docs(observability): record #801 mount baseline

[mdorman]

Summary

• record a certified five-run SQLite single-worker CSR mount baseline for Firefox and Chromium
• freeze the issue-local candidate list using the approved dual navigation/suite thresholds
• document why no candidate qualifies and close e2e/csr: cut CSR mount cost — size it from #794's boot breakdown and mount_to_settled_ms (#788 lever 4) #801 as no actionable win

Evidence

• 241 navigation records/run, 234 complete document boot totals/run, 11 settled values/run, zero dropped records
• Firefox suite baseline-only 3×SE eligibility floor: 49.4 s
• /app: 15 records/run, 13 complete cold mounts/run, zero mountToSettledMs values
• final Standards and spec-conformance reviews passed after findings were corrected

Verification

• cargo xtask precommit
• cargo xtask prepush
• five counterbalanced runs/browser of e2e-sqlite-{firefox,chromium}-single-worker

Closes #801