feat(npm): wire rebuild enqueue + tests (npm hosted step 12, part 5/5)

This change is split into 5 stacked MRs to keep each within review size. Each part targets the previous one (part 1 targets main); review and merge bottom-up, part 1 first.

Part 5 of 5 of docs/plans/2026-05-11-npm-hosted.md — Step 12: packument cache rebuilder (in-process).

Wires the inline-build paths to the rebuilder (packument_get.go and disttags.go call enqueueRebuild after serving a cache-miss; buffered.go gains the s22-observability registration marker), adds the test-only export seams in export_test.go, and lands the integration suite in packument_cache_integration_test.go. Also updates the Step 12 plan entry to match the shipped behavior.

Spec coverage

Acceptance criterion	Test
AC 29 — concurrent enqueues produce one rebuild per package	TestRebuildPackumentCache_Singleflight
Zero-versions package rebuilds cleanly	TestRebuildPackumentCache_ZeroVersions
Deterministic rendered bytes	TestRebuildPackumentCache_DeterministicBytes
Prior attachment left unreferenced (GC-eligible)	TestRebuildPackumentCache_PreviousAttachmentUnreferenced
Per-rebuild allocation is bounded	TestRebuildPackumentCache_MemoryBound
Panic safety — panic increments result="panic", process survives	TestRebuildPackumentCache_PanicSafety

• Plan: docs/plans/2026-05-11-npm-hosted.md
• Spec: S11-npm-hosted

Related to #130 (closed)

Stacked MRs (review/merge bottom-up)

• feat(npm): NpmPackageByID resolver (npm hosted ... (!712 - merged) • David Fernandez • 19.2
• feat(npm): metadata-file write methods (npm hos... (!713 - merged) • David Fernandez, Dzmitry (Dima) Meshcharakou • 19.2
• feat(npm): transactional cache upsert (npm host... (!714 - merged) • David Fernandez, Dzmitry (Dima) Meshcharakou • 19.2
• feat(npm): packument cache rebuilder core (npm ... (!715 - merged) • David Fernandez, Dzmitry (Dima) Meshcharakou • 19.2
• feat(npm): wire rebuild enqueue + tests (npm ho... (!716 - merged) • David Fernandez, Dzmitry (Dima) Meshcharakou • 19.2 👈