feat(npm): packument GET inline-build singleflight (S11 Step 11 part 4/4)

📦 Summary

Part 4 of 4 of the npm hosted packument GET handler (S11 Step 11). Adds the read-side singleflight so concurrent inline builds for the same package collapse to one:

• singleflight.Group keyed on (namespace_id, npm_package_id, kind), wrapping the inline build
• beforeSingleflightDo test seam (exported for tests via export_test.go) enabling a deterministic concurrency test
• Plan housekeeping: the Step 11 Files / Tests entries are reconciled to the shipped files, and the prose note referencing a non-existent npm_metadata_files.updated_at column is corrected (the packument supports ETag / If-None-Match only)

🧪 Tests

Unit: 10 concurrent inline-build requests collapse to exactly one build and return byte-identical 200s (deterministic via the seam barrier).

🗂️ Stack

Part 4/4, targets 10io/npm-hosted-step11-part3of4 (part 3/4). This is the final slice; the four parts together reproduce the validated, reviewed Step 11 change exactly.

Related to #129 (closed)

🔗 Stacked MRs (review/merge bottom-up)

• feat(npm): packument GET core serve (S11 Step 1... (!630 - merged) • David Fernandez • 19.2
• feat(npm): packument GET cache-hit serve (S11 S... (!631 - merged) • David Fernandez • 19.2
• feat(npm): packument GET conditional GET + cach... (!632 - merged) • David Fernandez • 19.2
• feat(npm): packument GET inline-build singlefli... (!633 - merged) • David Fernandez • 19.2 👈