✨ feat: emit app_startup.duration_ms alongside the seconds value (0.1.18)

[nitinstp23]

Summary

app_startup.duration carries seconds under an unsuffixed key — the same ambiguity that produced the 1000x ANR display bug fixed in 0.1.17 by renaming to anr.duration_ms. It bit again on snabbit ("Avg Cold Start 0.700 ms").

Every app_startup span now also carries app_startup.duration_ms. Unlike ANR, the seconds key is kept: RUM plugins before 0.1.34 read only app_startup.duration, so dropping it would blank their startup panels. Backends coalesce the ms key over seconds * 1000.

Changes

• src/core/attributes.ts: APP_STARTUP_DURATION_MS = 'app_startup.duration_ms' (pinned in attributes.test.ts).
• Emit sites: native cold (src/native/index.ts, reuses fbcMs), native warm (lifecycle.ts), web cold (Math.round(duration * 1000)) and web bfcache warm (0).
• docs/configuration.md: enableStartupTracking claimed a "hot" start that does not exist; now states the semantics and both keys.
• CHANGELOG.md: the 0.1.8 entry named app.startup.type / app.startup.duration_seconds, which were never emitted; corrected in place with a note. New 0.1.18 entry.
• Version 0.1.17 → 0.1.18 (package.json, lockfile root entries, scope.ts).

Verification

• vitest run: 294 passed. tsc --noEmit clean for both tsconfig.json and tsconfig.native.json. Prettier clean.

Related

• base14: RUM plugin + tenant MVs read app_startup.duration_ms with a seconds fallback
• scout-flutter 0.3.0: same key, plus process-start-anchored cold start