perf(i18n): read synced locale keys by direct lookup

[maxmad64bis]

⚠️ base-red inherited: #15306

Summary

Syncing UI keys across locales took 93.8 s for 2 new keys across 66 languages. The old path rebuilds the target node and scans it for the key, while the new path reads the key by direct lookup under the existing ownership check. The same timed run finished in 2.8 s with byte-identical output.

Related Issues

• No linked issue — reproduced on the release base

Validation

• Change type: i18n
• Focused tests and category gates from the golden path — focused suites green at the head (30/33 CI gates, 1/1 file tests, red-proof 1 cas); the full unit suite, Vitest, the coverage gate, and the production build run in CI.
• npm run lint — ESLint on the touched files is clean; the full run is red on the base (🔴 Release branch not green: release/v3.8.52 #15306).
• Reconciled with the current active release base; focused checks rerun afterward — rebased on the current release base; focused tests rerun.
• Production-code changes include a new or updated automated test in this PR — new regression test added in this PR covering the changed paths.

Tests Added Or Updated

• tests/unit/i18n-sync-ui-keys-merge.test.ts — covers merging 2 new keys byte for byte against the pre-change snapshot, dropping forbidden prototype keys, reporting no additions when in sync, and covering both shape-mismatch directions.

Coverage Notes

N/A — this PR touches only scripts, tests and changelog, none of src/, open-sse/, electron/ or bin/.

Reviewer Notes

• Behavior: locale sync output is unchanged when merging new keys, dropping forbidden keys, or finding the target already in sync. The timed sync run dropped from 93.8 s to 2.8 s with an identical checksum.
• Merge: shares the locale sync script with feat(i18n): translate several locales per request in sync-ui-keys #15562, after which feat(i18n): translate several locales per request in sync-ui-keys #15562 runs.