fix(approuter): preserve static/qa across content-rebuild swap (recurring QA navigator wipe)

[jung-thomas]

Problem

/tutorials-qa/ (the QA author-preview tutorial navigator) keeps disappearing on PROD (404). Confirmed on live PROD (authenticated): the QA backend is healthy (/tutorials-qa/_nav.json → 200, 2071 tutorials; /qa-search → 200), but the entire static frontend under static/qa/ (index.html, js, css) 404s.

Root cause (the persistent one)

rebuild-content.yml's "Assemble static content" step (L707–728) builds the tarball from hugo/public/* + the admin/scanner/explore/analytics/data-inspector dirs (added in #350) but omits static/qa. The approuter's /admin/rebuild handler then does an atomic whole-dir swap (approuter/server.js:380–383: rename static → old, rename new → static, rm old). Anything not in the tarball — i.e. static/qa — is deleted from the running pod on every full/catalog-only content rebuild. static/qa is shipped only by the full MTA deploy (root mta.yaml builds hugo/public-qa inline), so it never comes back until the next deploy.

Fix

Preserve deploy-only static subtrees across the swap. REBUILD_PRESERVE_DIRS = ['qa']; after the swap, carry each forward from the previous static dir only when the incoming tarball didn't ship it (so a future tarball that includes static/qa still wins). QA nav is independent of content rebuilds by design, so preserving (rather than rebuilding it every time) is the correct coupling.

Scope / caveats

• Cannot be verified by unit test or curl — needs a deploy + a content rebuild to exercise. Do not consider this "done" until verified on a deployed env (author could not deploy).
• This stops the wipe, but static/qa must be present for preserve to carry it — i.e. it relies on a deploy having shipped it. That gap (local deploy never builds hugo/public-qa) is closed by the companion PR (deploy-mta.cjs QA build + mtar guard). CI deploys already ship it via root mta.yaml.
• Alternative considered: add a QA fetch+build+assemble to rebuild-content.yml (matches the rebuild-content.yml clobbers admin-ui / scanner-ui / analytics-ui static dirs in approuter #350 include-in-tarball precedent). Heavier (a full QA build on every prod rebuild); preserve is lighter. Happy to switch if reviewers prefer consistency with rebuild-content.yml clobbers admin-ui / scanner-ui / analytics-ui static dirs in approuter #350.

Follow-ups

• Companion PR: local-deploy QA build + mtar-parity guard (static/qa/index.html present).
• Consider extracting the preserve loop to a tested helper (handler isn't currently exported).

Part of the "recurring missing QA navigator" cleanup.