feat(node-ci): composable sharded-Vitest + coverage-merge support for advanced consumers

[ArchonVII]

Summary

node-ci.yml@v1 is a single serial install → lint → typecheck → test → build job. It serves the common case well but cannot serve consumers that need sharded tests with merged coverage + threshold enforcement.

ArchonVII/pigafetta (see ArchonVII/pigafetta#1558) wants to consolidate onto the shared workflow, but migrating to v1 as written would regress its CI: lose 4-way test sharding (full suite unsharded risks the 25-min timeout / large slowdown), lose coverage-threshold gating, lose the tsc -b tsbuildinfo cache hook, and force branch-protection check-name churn.

Importantly, pigafetta does not need its doc-only fast lane, science-audit gate, or success-aggregation moved upstream — those stay caller-side. GitHub supports a caller matrix job invoking a reusable workflow, and the caller keeps its own change-detection + audit + ci-success aggregator. The genuine gaps are below.

Requested capabilities (a composable v2, or a dedicated vitest-sharded.yml)

• Configurable shard count input (e.g. test-shards: 4) running vitest run --shard=i/N --reporter=blob.
• Coverage blob upload + merge across shards (vitest --merge-reports) producing one combined coverage report.
• Threshold enforcement on the merged coverage (global + per-pattern), e.g. a config-path input, so per-shard runs skip thresholds and the merge job enforces once.
• Cache hooks — let the caller cache incremental build state (e.g. tsc -b tsbuildinfo) keyed independently of the lockfile.
• Timeout controls per job.
• Stable job outputs + check names so a caller's required-check aggregator can depend on them deterministically.

Why composable, not all-in-one

Keep pigafetta-specific change detection (doc-only fast lane), the science-audit drift gate, and the ci-success aggregator in pigafetta. The reusable piece should be the sharded-test + coverage-merge contract only, callable from a caller matrix job. This keeps the shared workflow general while letting the most advanced consumer adopt it without losing capability.

Refs

• Consumer (parked, blocked on this): ArchonVII/pigafetta#1558
• Current pigafetta ci.yml: sharded test (1–4) + coverage merge job with VITEST_SKIP_THRESHOLDS gating; separate changes (paths-filter), audit, and ci-success jobs.

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