docs(loops): document the server-rendered loop page query parameter

[asachs01]

Summary

An infinite-mode loop already server-renders any page of its own results from ?loop_<loopNodeId>_page=N. loopPageQueryKey builds the name (server/publish/loopPrefetch.ts:177), readPageNumber reads it (:204), and publicRouter.ts:224-240 deliberately routes any request carrying it past the Layer A disk artefact into a live Layer B render.

The parameter is not undocumented — but it is documented only as a cache-invalidation input, in docs/architecture.md:240, docs/server.md:487, docs/features/publisher.md:19,384,474 and docs/reference/architecture-tests.md:202. All six describe how it affects the fast path, none mention that it renders a different page.

docs/features/loops.md — the doc a page author actually reads, and the one that describes the pagination modes — never mentions it. Its "Pagination" section instead recommends a manual workaround for precisely the case the parameter already covers:

For static multi-page navigation (no JS required):

• Use separate base.loop nodes with an offset filter — one per "page" — and static links between pages.

This adds a ### Deep-link to a loop page subsection next to that one, and points the existing bullet at it.

What it documents

The mechanism, plus the four constraints that bound it — deliberately, so the section cannot be read as promising a numbered paginator:

Constraint	Source
honoured only when pagination: 'infinite', silently ignored otherwise	loopPrefetch.ts:229
never applies to contextual sources (entry.field)	loopPrefetch.ts:292-299, renderLoop.ts:155-159
a page past the end renders the empty string — no wrapper, no attributes	renderLoop.ts:78-80
no item or page total is exposed anywhere, so "Page 3 of 12" is not derivable	renderLoop.ts:111-116, server/handlers/cms/loop.ts:131,169

src/modules/base/loop/index.ts:17-23 says numeric pagination will live in a future base.pagination module, so per docs/CONVENTIONS.md rule 8 nothing here describes that as existing.

Also in the same change, since they belong to the same section:

• a "Forbidden patterns" row for the silent-ignore case (rule 6);
• canonicalRenderQuery.test.ts and static-artefact-served-before-render.test.ts added to the "Gate tests" list, and publicRouter.ts to the source-of-truth list (rule 4).

Verification

Docs-only — no code changed, so no new test. Claims were checked against the source at 6b055cf7 and against the existing suites:

$ bun test src/__tests__/publisher/canonicalRenderQuery.test.ts \
           src/__tests__/publisher/loopRender.test.ts \
           src/__tests__/architecture/static-artefact-served-before-render.test.ts
 20 pass
 0 fail
Ran 20 tests across 3 files. [187.00ms]

and the canonicalisation claim directly:

"?utm_source=x&gclid=y"          -> ""
"?loop_ABC_page=2"               -> "?loop_ABC_page=2"
"?utm_source=x&loop_ABC_page=2"  -> "?loop_ABC_page=2"
"?loop_B_page=3&loop_A_page=2"   -> "?loop_A_page=2&loop_B_page=3"

File is 531 lines, within the ~600 cap. No nav registration needed — docs/README.md indexes files, not subsections.

Note

#350 also touches docs/features/loops.md, but adds a subsection around line 265; this one lands in the Cookbook at 460+. They should not conflict.

Happy to cut the "Forbidden patterns" row or the gate-test lines if you would rather keep this to the new subsection alone.