serve Atom feeds for the site, a section and a tag

Membership is a publication date, not a type declaration (ADR-0043). The parked
feed shape said "every type declared primary", which would have made feeds wait on
declared types a third time — but the thing that distinguishes a feed item is
already on disk. Pages and section landings drop out because they have no date,
which is the right reason. The parked idea stays parked with a sharper trigger:
someone wanting a *dated* bundle kept out.

Built with encoding/xml from typed structs, never a template: XML in html/template
is escaping for the wrong grammar, and that is a correctness trap rather than a
matter of taste.

A bug the evidence found, older than feeds: content.URL("", lang) built "//", so a
whole-site feed's id and alternate link were https://khosra.example// — every
entry identity wrong in every reader. The root is "/" now, with a test, and the
hand-built "/" the resolver carried for the same reason can follow later.

Two counters re-scoped rather than incremented, the same way transforms was:

Views now counts *per-bundle selection* — the thing architecture.md means by the
View layer, still at zero consumers. Output formats are not it: HTML, sitemap XML
and Atom are three functions with nothing to share, so an interface over them
would have one member and no leverage.

Effects stays at 1. A feed is generated per request like the sitemap, so it is not
a second Effect and the runner is not yet due — the next thing that writes files
off the request path is.

docs/state.md

@@ -20,15 +20,17 @@ If this file disagrees with the code, the code is right and this file is a bug.
 | `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/feed.go` | Atom for the site, a section or a tag, from dated bundles via one Query (ADR-0043) | 125 |
 | `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`, `-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 |
+| `*_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, feeds, 404 | 2334 |
 
 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, generated derivatives under `/derived/`, plus `/robots.txt` and `/sitemap.xml`.
+URL, generated derivatives under `/derived/`, Atom feeds per site,
+section and tag, 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
@@ -48,9 +50,9 @@ this change*.
 | Counter | Now | Extraction due at | What it buys |
 |---|---|---|---|
 | Render transforms — **page-level only** | 0 | **3** | Stage pipeline (ordered `func(ctx,*Page) error`). Parse-phase work does *not* count and must not: goldmark's extender list is already an ordered pipeline for it, so typography, shortcodes and widows compose there (`cmd/khosra/wire.go`) and a second pipeline beside it would be pure duplication. This counts transforms over the assembled page, which nothing hosts yet — OpenGraph and JSON-LD (queue 15) are the first candidates |
-| Routing cases | 8 | **2** — done | Resolver at `internal/web/resolve.go`: bundle, language prefix, pagination, tag, section-narrowed tag |
+| Routing cases | 9 | **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 |
+| Views — **per-bundle selection only** | 0 | **2** | The View layer `architecture.md` describes: `view:` in frontmatter choosing a presentation, resolved through the cascade. Nothing selects a view yet. *Output formats* are counted separately and are not it: HTML, sitemap XML and Atom are three functions with nothing to share — an interface over them would have one member and no leverage |
 | 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 |