record ADR-0039: site.yaml, and only its site level

The human asked for YAML site config alongside the -base flag, which is the
trigger the parked settings cascade was waiting for — but only for its top level.
So this adopts the site level and leaves section and bundle resolution parked,
with their trigger unchanged: the first section-level override with a reader.
The parked idea now says so, rather than reading as untouched.

Absolute URLs are the forcing function. A canonical link, an hreflang, an
OpenGraph tag and a sitemap entry all need the site's own origin, which the engine
cannot infer from a request it may be serving behind any proxy.

Precedence stated in the ADR: -base beats the file, so a staging host needs no
edit to content.

docs/decisions.md

@@ -498,3 +498,24 @@ the same Origin. Expensive — a page assembled from parts that themselves compo
 `PhaseLoad` loses its example: includes turned out to be parse-phase, not load-phase.
 Revisit if: composing partials out of partials becomes a real need. That is textual splicing before parsing,
 and it wants the Stage pipeline's load phase rather than a second mechanism here.
+
+## ADR-0039 — `site.yaml` declares the site; the cascade below it stays deferred
+Date: 2026-07-30 · Status: accepted (adopts the *site* level of the parked settings cascade in
+`ideas/deferred-decisions.md`, and only that level)
+Decision: a site root may hold `site.yaml` carrying declared site-level keys — `base` and `title` today. It
+is read once at startup: absent is fine, since a bare site root still serves, but malformed is a fatal
+startup error, because unlike one bad bundle it misconfigures every page. `-base` overrides the file when
+given, so a staging host needs no edit to content. Section-level and bundle-level resolution are *not* built:
+frontmatter already covers a bundle, and nothing reads a section override yet.
+Why: absolute URLs are the forcing function — a canonical link, an `hreflang`, an OpenGraph tag and a sitemap
+entry all require the site's own origin, which the engine cannot infer from a request it may be serving
+behind any proxy. A flag alone would work, but the origin is a property of the site rather than of one
+invocation, and it belongs in the site's own git history next to the content it describes. Building only the
+site level keeps the earn-it rule honest: the cascade's cost is resolution per bundle, and nothing yet asks
+for it.
+Consequence: cheap — one file, two keys, read once, and a site that declares nothing keeps working with
+relative links. Expensive — there is now a second place a setting can come from, so precedence has to be
+stated wherever a key is documented; and the set of keys must stay declared or `site.yaml` becomes unbounded
+config, which is the failure the parked cascade already warns about.
+Revisit if: a section wants to set a policy for everything beneath it. That is the parked cascade arriving
+for real, and its trigger is unchanged: the first section-level override with a reader.

ideas/deferred-decisions.md

@@ -57,6 +57,10 @@ new field on every type.
 
 ## Settings cascade
 
+Status: partly adopted → the **site** level is built as `site.yaml` (ADR-0039). The section and bundle levels
+stay parked: frontmatter covers a bundle, and the trigger for the rest is unchanged — the first section-level
+override with a reader.
+
 Deferred because frontmatter alone covers the MVP; a cascade earns itself when a section-wide policy is actually wanted.
 
 ## Settings cascade: site → section → bundle