fix(quality): clear base-reds on release/v3.8.51 — radar i18n EN keys + stream gates

[maxmad64bis]

Summary

Base-red fix (own PR per AGENTS.md § Base-green check — never inside the feature branch inheriting the red). Drains the 4 red gates on tip 438db55 (release/v3.8.51) inherited by #12214/#12317.

• src/i18n/messages/en.json — add radarPage.colLimits / trainsOnPrompts / trainsOnPromptsHelp (values = English text of the __MISSING__ sentinels left by feat(radar): show the rate limits and training disclosure the feed already sends #12320). Without EN, deepMergeFallback had nothing to substitute and RadarCatalogTable.tsx:223/325/327 failed the key scanner.
• src/i18n/messages/vi.json — same trio without __MISSING__ (vi requires zero __MISSING__, i18n-vi-completeness).
• tests/unit/stream-passthrough-usage-estimation.test.ts — drop dead collectSSE helper (3 no-unused-vars, fix(sse): estimate usage in passthrough stream even with include_usage #12151, same as fix/release-v3.8.51-eslint-unused-collectsse 3b1520b).
• tests/unit/stream-utils.test.ts — forwards OpenAI usage-only empty choices chunks expected trailing choices:[] to be forwarded; fix(sse): estimate usage in passthrough stream even with include_usage #12151 now injects usage.estimated:true on stop and drops the trailing (passthroughForwardedUsage). Update to estimated:true / doesNotMatch choices:[] (tradeoff documented in stream-passthrough-usage-estimation).

Related Issues

• Related to feat(providers): expose usage-supported in provider plugin manifest #12214, feat(auto-combo): declare observed reliability as a scoring factor #12317 (unblocks inherited UNSTABLE)

Validation

• Change type: i18n / other (quality gates)
• Focused tests and category gates from the golden path — npm run i18n:check-ui-coverage PASS, i18n:check-value-drift FAIL pre-existing on tip 438db55 (2 EN rewrites, 35 stale — untouched), check:dashboard-typecheck FAIL pre-existing on tip (2 jsonView.ts/useTimestampTitles.ts TS2307 on clean tip)
• Focused unit tests rerun after rebase: i18n-missing-placeholder-fallback 4/4, settings-i18n-keys 10/10, stream-passthrough-usage-estimation 6/6, stream-utils 40/40, i18n-vi-completeness 5/5 green in this worktree
• npm run lint — No new ESLint warnings green on this PR (was red on tip due to collectSSE)
• Reconciled with current active release base (438db55); no rebase pending
• Production code changes — no runtime change; updated tests in this PR cover the behavior

Tests Added Or Updated

• tests/unit/stream-passthrough-usage-estimation.test.ts — drop dead helper (stays 6/6)
• tests/unit/stream-utils.test.ts — update forwards OpenAI usage-only empty choices chunks for post-fix(sse): estimate usage in passthrough stream even with include_usage #12151 behavior (estimated vs trailing tradeoff)
• tests/unit/i18n-missing-placeholder-fallback.test.ts and tests/unit/settings-i18n-keys.test.ts — untouched, green again via en.json
• tests/unit/i18n-vi-completeness.test.ts — green again via vi.json

Coverage Notes

• No change in src//open-sse//electron//bin/ beyond src/i18n/messages/*.json (i18n data). No coverage impact.

Reviewer Notes

• No migration, no flag, no runtime change. i18n:check-value-drift and check:dashboard-typecheck stay red on tip and here — pre-existing, out of scope for this base-red (same as prior fix/basereds). exclusive-connection-leases flaked once as database is locked (race, not touched).

Closed as duplicate of #12327 (1586476) which landed the same scope more completely (real stream.ts fix + pt-BR alongside vi).

[maxmad64bis]

Closing — duplicate of #12327 which merged in the meantime (1586476 fix(release): drain the 2026-09-01 base-red window).

Same scope (radarPage EN + stream gates) handled more completely there: estimated usage leak in passthrough fixed in stream.ts (not just the test), plus pt-BR alongside vi. This PR (7a3f849) would only re-apply half the stream-utils side and overwrite the deeper fix.

Base is green again — #12214 and #12317 benefited.