Add deterministic search for the currently released documentation archive

## User impact and evidence

Production documentation must not mix unreleased v0.14 search data with the immutable v0.13 release archive. The recovery therefore serves direct released routes and excludes the unreleased Pagefind index from the production artifact. The v0.13 archive was sealed without a deterministic search index.

## Why this is non-blocking now

The restored homepage, Start routes, and exact task links provide direct navigation for the governed first-country journeys. Copying the v0.14 index into v0.13 would create a mixed-release public surface. Search becomes blocking only if independent human acceptance shows direct navigation is insufficient or a future release gate explicitly requires released-site search.

## Owner

Registry Stack documentation and release maintainers.

## Prerequisites

- A deterministic index format and build procedure whose bytes can be included in the release docset digest.
- A release-boundary decision covering canonical URLs, archived content, and stale-index rejection.
- Retained fresh-reader navigation evidence from the corrected direct routes.

## Reconsideration trigger

Promote this work when a fresh-reader exercise records inability to find the governed task through direct navigation, or before a release whose acceptance criteria require search over the released archive.

## Expected verification

- Search results contain only the selected immutable released docset.
- Repeated builds from the same frozen source produce identical index bytes.
- The index digest is covered by the release archive and lock.
- Unreleased, draft, and other-version content cannot appear in results.
- Missing or mismatched indexes fail closed without breaking direct navigation.
- Production staging never copies a preview index into a released archive.

Start by resolving the deterministic index format and build procedure, then the release-boundary decision for canonical URLs, archived content, and stale-index rejection. Verify that repeated frozen-source builds match, the digest is covered by the release archive and lock, excluded content cannot appear, failures are closed safely, and staging never copies a preview index into a released archive.

Written by the indexing model from the issue text.