perf(translation): E-04 E-06 share in-flight translations and translate each caption once

[navidshad]

What

• E-04 — share translations already in flight. TranslateService keeps a map of requests on their way, by cache key. If the same call comes in before the first one answers, it waits for that request and gets the same answer. Only one network call goes out. The entry is removed when the request settles. A failure reaches every waiting caller and is never cached.
• E-06 — translate each caption once. New helper src/subtitle/helpers/caption-translation-gate.ts, used by web_netflix/components/Subtitle.vue and _support-template/components/Subtitle.vue. With "translate whole caption" on:
  • It waits about 150 ms for the caption to settle.
  • It skips the request when the joined caption equals the one already translated on screen (for example, a caption that blinks off and back on).
  • A new caption clears the old translation at once, so the old text is not shown under the new caption while it settles.
  • A failed caption can be sent again.
  • Turning the option on still translates at once (no wait).

Why

The cache was filled only after the answer came back, so two calls for the same text at the same time both went out. The caption watcher sent a request on every caption change, sometimes more than once for one caption. Work list: subturtle-doc docs/tech/analytics-fixes-implementation.md (rows E-04, E-06).

How tested

• New unit tests: in-flight cases in tests/translate.service.test.ts (one request for two calls, detailed too, different keys not shared, failure reaches both callers and is not kept) and tests/caption-translation-gate.test.ts (settle delay, skip same caption, A → B → A, send now, retry after failure, cancel).
• yarn typecheck: clean.
• yarn test: 41 files, 369 tests pass.
• yarn build: OK.
• The repo has no lint script.
• Not checked on real Netflix (the caption path cannot run in our test setup; see CLAUDE.md "Still manual").

Dependencies

• None. The translate.service.ts change is kept small and away from the lines fix(analytics): E-01 E-05 send the install id and trigger with translation calls #58 (E-01/E-05) changes. I checked with git merge-tree: this PR merges cleanly with fix(analytics): E-01 E-05 send the install id and trigger with translation calls #58, feat(analytics): E-02 track installs and updates and open the welcome page #59 and feat(analytics): E-03 track the save-login prompt and login results #60 in any order.
• E-07 (keep translations across page loads) is not in this PR. It builds on E-04 later.

🤖 Generated with Claude Code

[navidshad]

Replaced by #62, which combines all extension rows (E-01 to E-06) into one PR.