ci: add serial cross-platform reference runs

.agents/notes/implemented/process/2026-07-06-parallel-github-ci-gates.md

@@ -12,13 +12,13 @@ The artifact boundary remains load-bearing. `publint`, `verify-node-next-types`,
 
 ## Decision
 
-[CI](../../../../.github/workflows/ci.yml) bounds every non-Windows job to one minute and every Windows job to three minutes. The timeout is an executable regression ceiling; the lane design leaves headroom below it rather than treating a timeout as normal control flow.
+[CI](../../../../.github/workflows/ci.yml) treats one minute for non-Windows jobs and three minutes for Windows jobs as observed performance targets, not cancellation deadlines. Hosted-runner variance should leave complete timing evidence and useful failure logs instead of cancelling an otherwise-correct gate. The [serial cross-platform CI reference](2026-07-21-serial-cross-platform-ci-reference.md) independently runs the complete unsharded primary Node aggregate on Linux, macOS, and Windows so the optimized lane inventory is not its own completeness oracle.
 
 [scripts/run-gates.ts](../../../../scripts/run-gates.ts) remains the common bounded scheduler, but GitHub supplies explicit shard names for the expensive gate families. [scripts/static-shards.ts](../../../../scripts/static-shards.ts) partitions static gates into foundation, documentation-type, API-contract, catalog, prose, documentation-projection, and documentation-build lanes and rejects a missing or duplicate gate assignment. Lint uses disjoint package-source, package-test, and repository-complement lanes; the complement still starts from `.` so a new top-level lint target cannot disappear between shards, and it owns the single cross-file duplication run. [scripts/coverage-shards.ts](../../../../scripts/coverage-shards.ts) assigns every workspace package to exactly one source-coverage lane; its test expands the live package tree, so a new package makes CI red until it has an owner. Each coverage lane includes only its owned source files, repeats the exhaustive companion topology test, and runs without a preceding build because the complete coverage suite passes from a tree with every generated `lib/` removed.
 
 Snapshot replay uses two explicit multi-file lanes and six scenario partitions of the large ACP file. [scripts/snapshot-shards.ts](../../../../scripts/snapshot-shards.ts) owns that inventory, and its test discovers every file admitted by the snapshot config so a new file cannot land outside CI. Each snapshot job builds the shipped runtime while its Linux runner installs bubblewrap from the hosted image's existing package index, then runs only its assigned replay surface. CI explicitly retains the suite's bounded concurrency of five subprocesses instead of clamping it to the runner's two logical CPUs, because replay spends most of its time waiting on child protocol I/O. Fixture guards still inspect the complete ACP scenario table in every partition. Static, lint, coverage, and snapshot sharding changes only GitHub scheduling: the ordinary local package scripts still run their complete suites.
 
-Cold standalone documentation typechecking rebuilds the complete project-reference graph, so a dedicated documentation-type lane builds once and checks Markdown blocks against those declarations. The Linux documentation lane uses VitePress's MPA build to retain page rendering and dead-link validation inside the one-minute budget; the blocking Windows build also produces the normal production SPA, preserving the shipped-site check under its three-minute budget.
+Cold standalone documentation typechecking rebuilds the complete project-reference graph, so a dedicated documentation-type lane builds once and checks Markdown blocks against those declarations. The Linux documentation lane uses VitePress's MPA build to retain page rendering and dead-link validation within the observed non-Windows target; the blocking Windows build also produces the normal production SPA, preserving the shipped-site check within the observed Windows target.
 
 Artifacts use two lanes: one metadata lane for `publint`, NodeNext declarations, and compiled invariant loading, plus one built-bin smoke lane. Each lane produces its own build before its consumers. Repeating the short build costs runner minutes but avoids an upload/download dependency and keeps each job's critical path bounded.
 
@@ -26,11 +26,11 @@ Artifacts use two lanes: one metadata lane for `publint`, NodeNext declarations,
 
 Compatibility lanes run the source worker and Zstandard runtime smokes on every advertised Node line. TypeScript checks the source graph once in a dedicated primary Node 24 lane; repeating the same compiler analysis in runtime compatibility jobs added time without runtime-specific signal.
 
-The workflow caches the pnpm store, preserves native PowerShell for Windows measurements, and retains one aggregate `all checks passed` status for branch protection. Windows reuses the three exhaustive lint partitions and groups foundation/catalog/prose plus documentation-type/API-contract gates behind shared runner setups; only scheduling differs from the Linux partitions. Windows build remains blocking; the wider Windows static, lint, and artifact matrix remains observational while carrying the same three-minute ceiling.
+The workflow caches the pnpm store, preserves native PowerShell for Windows measurements, and retains one aggregate `all checks passed` status for branch protection. Windows reuses the three exhaustive lint partitions and groups foundation/catalog/prose plus documentation-type/API-contract gates behind shared runner setups; only scheduling differs from the Linux partitions. Windows build remains blocking, while the wider Windows static, lint, and artifact matrix remains observational.
 
 ## Alternatives considered
 
-- **Keep the broad lanes and raise timeouts** - minimizes workflow YAML, but it preserves the measured multi-minute feedback loop and offers no regression budget.
+- **Keep the broad lanes** - minimizes workflow YAML, but it preserves the measured multi-minute feedback loop.
 - **Run every leaf gate as a separate GitHub job** - maximizes fan-out, but short generators and prose checks would spend more time preparing a runner than checking the repository.
 - **Upload one build to artifact consumers** - avoids repeated compilation, but upload/download and dependency scheduling lengthen wall time; the clean build is short enough to repeat inside bounded lanes.
 - **Keep package-manager packing in both publication gates** - delegates inventory selection to pnpm, but repeats more than 200 package-manager processes. The manifest structural gate plus publication-view fixtures make the optimized inventory contract explicit and fail on an on-disk but unpublished dependency.