feat(ci): build Japanese as well as English on PRs

[ChronosSF]

Expected to be red until #873 merges. Exactly one job — build-and-verify (angular, jp) — fails, at the Build Angular Unique (jp) step, on the missing JP carousel image. That is the bug this PR is designed to catch, so it failing here is the change working. See Verification below.

Problem

CI built English only. The matrix covered platforms, not languages, and both build steps used the default (EN) scripts. Nothing in PR validation ever exercised Japanese, so JP-only breakage reached vnext and was found by a deploy instead of by a check.

An EN build is not a proxy for a JP one, because the two do not resolve content identically. docs/angular/astro.config.ts maps @xplat-images to src/content/<lang>/images, so an image present in EN does not satisfy a JP import — which is exactly how #873 happened, and what #842 asks for a check against.

Change

Add a lang axis to the existing matrix, taking build-and-verify from 4 jobs to 8:

matrix:
  platform: ['angular', 'react', 'webcomponents', 'blazor']
  lang: ['en', 'jp']

Each build step picks the language variant by suffix, so the EN legs run exactly the commands they run today and the JP legs run the same scripts the deploy workflows use:

lang	xplat step	Angular-unique step
en	npm run xplat:build:<platform>	npm run angular:build
jp	npm run xplat:build:<platform>:jp	npm run angular:build:jp

DOCS_TOC_STRICT: 'true' applies to both languages, so a toc.json entry pointing at a missing file now fails the JP build too, not just EN.

Also sets fail-fast: false. With two languages in the matrix the default would let a JP failure cancel the EN jobs, hiding results still worth seeing.

Verification

Run locally against this matrix:

• DOCS_TOC_STRICT=true npm run xplat:build:{angular,react,webcomponents,blazor}:jp — all four pass. No pre-existing JP strict-TOC problems, so nothing else here is lying in wait.
• DOCS_TOC_STRICT=true npm run angular:build:jp — fails on vnext with the carousel ImageNotFound; passes on fix(jp/angular): complete the carousel xplat sync for Japanese #873's branch (421 pages).

So once #873 is in, this goes green on all 8. It will need a merge from vnext after that to pick the fix up.

Cost

Doubles build-and-verify from 4 jobs to 8. They run in parallel, so wall-clock is roughly unchanged; runner minutes roughly double.

Partly offset by the concurrency group added here: pushing to a PR now cancels the run it supersedes, instead of leaving all eight jobs to finish for a result nobody reads. Grouping is on head_ref so other PRs are untouched, and cancelling is safe because these jobs only build — nothing is published or deployed, so a job killed mid-step leaves no partial state. The one tradeoff is that a run cancelled mid-flight gives partial logs rather than a final failure summary.

If cost still matters after that, the remaining lever is narrowing the JP axis to Angular only — Angular is where the per-locale @xplat-images resolution actually bites, and the xplat JP builds are the cheaper half of the value.

Related

• fix(jp/angular): complete the carousel xplat sync for Japanese #873 — the JP carousel breakage that motivated this, and the reason this PR is currently red.
• CI: detect missing localized documentation image imports #842 — "CI: detect missing localized documentation image imports". This satisfies that acceptance criteria by building the real thing rather than adding a bespoke checker: the failure names the importing topic, platform/language and missing path, and it covers image deletions as well as topic edits.
• Adjust deployment tasks to trigger angular on xplat changes #402 / fix(ci): trigger Angular deploys on xplat changes #874 — the deploy-trigger gap that let fix(jp/angular): complete the carousel xplat sync for Japanese #873 sit undetected. Complementary: fix(ci): trigger Angular deploys on xplat changes #874 makes sure the deploy runs, this makes sure the problem is caught before merge.

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