generate sized derivatives ahead of the request

A pass over the content at startup writes three widths per picture into a cache
outside the site root, named by the source's content hash and the width (ADR-0042).
Idempotent by construction: a rerun stats and skips, an edited picture takes a new
name, and nothing stale can be served under an old one. Restarting the evidence
site made 0 derivatives the second time, as it should.

Ahead of the request rather than during it, because resampling is felt and there is
no page cache yet to hide it. Outside the site root, because the engine reads that
directory and must not leave generated files in somebody's content git — a lost
cache costs one startup pass and no correctness.

Markup now carries the original as src, the derivatives as srcset closed by the
original at its own width, and width/height from the original — which retires most
of the latent row about the output floor; only a gallery's alt is still empty, and
a filename cannot supply that.

Two things the work itself decided:

`Fragment.Items` became `Fragment.Pictures`, ADR-0037's own revisit trigger. Items
had one consumer, so widening it beat adding a second list beside it.

"A browser can show it" and "we can resample it" are different questions, and
conflating them nearly deleted content: an SVG has no decoder here, so a single
predicate would have dropped SVGs from galleries silently. Undecodable and
unsupported pictures are now rendered as they are, without a size or a srcset.

docs/state.md

@@ -15,20 +15,20 @@ If this file disagrees with the code, the code is right and this file is a bug.
 | `internal/render/render.go` | goldmark with the typographer, per-kind template sets with site override, the `Partial`/`Origin` seams features render and resolve through, `Page`/`List`/`Sequence`/`head` | 409 |
 | `internal/render/chrome.go` | the engine's own words: phrase table, month names, digits, and the `t`/`num`/`day` template funcs (ADR-0034) | 105 |
 | `internal/render/templates/` | reference theme: `base.html`, `page.html`, `list.html`, `shortcodes.html`, `theme.css` (ADR-0026) | — |
-| `internal/ext/shortcodes/` | first feature: `{{< name key="value" >}}` block parser and node renderer, rendering through a theme fragment (ADR-0036). `figure`, `gallery`, `include` | 315 |
+| `internal/ext/shortcodes/` | first feature: `{{< name key="value" >}}` block parser and node renderer, rendering through a theme fragment (ADR-0036). `figure`, `gallery`, `include`, plus the derivative pass and picture inspection (ADR-0042) | 534 |
 | `internal/ext/widows/` | second feature: joins the last two words of a paragraph or heading with a non-breaking space, over the tree so code spans are safe | 108 |
 | `cmd/khosra/wire.go` | the only list of enabled features (`extensions.md`) | 20 |
 | `internal/web/resolve.go` | URL → (key, lang, page, tag) or a canonical redirect: language prefix, `/en/…` fork guard, pagination, tags, trailing slash | 112 |
 | `internal/web/asset.go` | files inside a bundle's own directory, looked up through the owning bundle so visibility can only ever inherit (ADR-0024) | 58 |
 | `internal/web/discover.go` | `/robots.txt` and `/sitemap.xml`, absolute and only with a declared base (ADR-0039) | 74 |
 | `internal/web/web.go` | handler: resolve, look up with fallback, section and tag listings, sequence, `/static/` (misses and refusals alike answer 404), degrade on failure | 152 |
-| `cmd/khosra/main.go` | flags (`-site`, `-addr`, `-base`), wiring, startup — the only place things are assembled | 60 |
-| `*_test.go` | table-driven, one file per source file; symlink escape (content and static), canonical paths, language fallback, aliases, pagination, tags, sequences, chrome, typography, shortcode escaping, galleries, includes, partials, widows, site settings, absolute URLs, robots, sitemap, slug routes, bundle assets, 404 | 2050 |
+| `cmd/khosra/main.go` | flags (`-site`, `-addr`, `-base`, `-cache`), wiring, startup including the derivative pass — the only place things are assembled | 88 |
+| `*_test.go` | table-driven, one file per source file; symlink escape (content and static), canonical paths, language fallback, aliases, pagination, tags, sequences, chrome, typography, shortcode escaping, galleries, includes, partials, widows, site settings, absolute URLs, robots, sitemap, slug routes, bundle assets, derivatives, 404 | 2170 |
 
 Serves a bundle at `/{section}/{slug}/` — the slug derived, or declared in frontmatter without moving the
 key (ADR-0035) — a paginated listing per section, tag listings global and
 section-narrowed, sequence navigation and a series archive on any nested bundle, `static/` verbatim, a directory bundle's own files under its
-URL, plus `/robots.txt` and `/sitemap.xml`.
+URL, generated derivatives under `/derived/`, plus `/robots.txt` and `/sitemap.xml`.
 Chrome text, dates and digits render in English or Bengali; authored text is untouched but for typographic
 smoothing and widow prevention (ADR-0034). This repo holds engine source only — the site root is external and passed with
 `-site` (ADR-0011). `site.yaml` declares `base` and `title`; with a base, canonical, hreflang and OpenGraph
@@ -38,7 +38,7 @@ Frontmatter the parser lifts today: `title`, `date`, `tags`, `aliases`, `order`,
 `content-model.md`'s table — including `draft` and `type` — lands in `Extra` unread, so that table
 is the accepted format, not a list of what runs.
 
-Dependencies: three, all allowlisted — `goldmark`, `golang.org/x/text`, `gopkg.in/yaml.v3`.
+Dependencies: four, all allowlisted — `goldmark`, `golang.org/x/text`, `golang.org/x/image`, `gopkg.in/yaml.v3`.
 
 ## Counters — the earn-it authority
 
@@ -51,13 +51,13 @@ this change*.
 | Routing cases | 8 | **2** — done | Resolver at `internal/web/resolve.go`: bundle, language prefix, pagination, tag, section-narrowed tag |
 | Collection pages | 4 | **1** — done | Query primitive: `content.Query{Section, Tag, Lang}` + `Site.Run`. The fourth — a series archive — resolves through `Site.Sequence` instead: membership is structural and the sort ascends, so it shares the index but not the Query |
 | Views / output formats | 2 | **2** — due | Two template sets exist (bundle, listing); the View layer is Arc 2's third item |
-| Effects | 0 | **2** | Effect runner + trigger wiring (change / schedule / demand) |
+| Effects | 1 | **2** | Effect runner + trigger wiring (change / schedule / demand). The first is the derivative pass (ADR-0042), called straight from `cmd` at startup — one call needs no runner, and startup is the only change signal until queue 21 |
 | Extensions | 2 | **3** | Extension registry (`extensions.md`). The wire file arrived with the first feature rather than the registry — `cmd/khosra/wire.go`, one line, no struct |
 | Interface implementations | — | **2** | The interface itself |
-| Non-stdlib dependencies | 3 direct | budget in `scripts/budgets.env` | — |
+| Non-stdlib dependencies | 4 direct | budget in `scripts/budgets.env` | — |
 
-Allowlist, all three imported: `goldmark` (markdown), `golang.org/x/text` (NFC, ADR-0015),
-`gopkg.in/yaml.v3` (frontmatter, ADR-0020).
+Allowlist, all four imported: `goldmark` (markdown), `golang.org/x/text` (NFC, ADR-0015),
+`gopkg.in/yaml.v3` (frontmatter, ADR-0020), `golang.org/x/image` (resampling and WebP, ADR-0040).
 
 ## Latent items — known, deliberately unfixed
 
@@ -70,7 +70,7 @@ with a stated reason. A list nothing drains is a graveyard of known defects.
 | No mechanical check that the counters are *correct* | Accepted at the Arc 1 boundary: the coupling gate makes forgetting them impossible, which is the real failure mode, and checking the values needs code to count | The first page-level transform (queue 15), now that the transform counter means something narrower |
 | No mechanical gate on the untrusted boundary (ADR-0003) | Scheduled to Arc 3: nothing untrusted is read yet. Half of it is now mechanical — `verify.sh` rejects `WithUnsafe`, so authored Markdown cannot become HTML — but there is still no check that a *future* untrusted source stays out of shortcode and template evaluation | The comment path — a test that untrusted input reaches no shortcode or template evaluation |
 | `date` stays in `Extra` after being lifted onto `Bundle.Date`, unlike `title`, `aliases`, `tags` and `order`, which are deleted | Spotted while adding `order`; the theme contract says `Extra` holds what the parser does not name, so one of the two is wrong. Harmless today — a template reading `.Extra.date` gets the raw YAML value | Whatever next reads `Extra` generically: feeds (queue 14) or `check` (17) |
-| The reference theme's images carry no `width`/`height`, and a gallery's carry no `alt` — below the output floor `conventions.md` states | Nothing can supply them yet: dimensions need the image read, and a filename is not alt text. An empty `alt` is at least honest about a picture nothing describes | Image derivatives (queue 13) compute dimensions; structured gallery items with captions land with them (ADR-0037's revisit note) |
+| A gallery's images carry no `alt` | `width`/`height` now come from the original (ADR-0042), so only alt text is missing, and a filename does not supply one. An empty `alt` is honest for a picture the page has already introduced | Captions per gallery entry — a sidecar or a frontmatter list — if the reference theme ever needs them |
 | Sequence resolution rescans the index on every bundle request — two passes over every key, each doing a `Lookup` | No cache exists anywhere yet, and a site of this size resolves in microseconds. Measuring first is the rule (queue 16) | The page cache (queue 16), which is the thing that makes the cost visible |
 
 ## Open questions