npm hosted Step 11: packument GET (full + abbreviated) with inline-build

GET .../{pkg} with Accept dispatch (vnd.npm.install-v1+json -> kind=2, else kind=0). Serves the cache blob via BlobStore.OpenBlob when fresh, else inline-builds from npm_versions + npm_tags (excludes soft-deleted, projects abbreviated fields for kind=2). Vary-by-visibility cache headers (spec departure), Vary: Authorization, Accept; conditional GET -> 304; handler-side singleflight. (enqueueRebuild is wired by Step 12.)

Acceptance: AC 12, 13, 14/15 (integration halves), 27/28 (partial; completed by Steps 12/16), 34, 58; cache/304/Vary behaviors; singleflight one-build. AC 56 deferred. Files: packument_get.go + unit + integration tests. Depends on: Steps 4, 8, 25. External: S06 Step 1; spec vary-by-visibility amendment.

Plan: Step 11. Spec: docs/specs/S11-npm-local.md.