build(repo): typecheck the test sources and close the last Phase 1 items

[evk-soft]

What lands and why

The last two Phase 1 deferred items — packet 4B step 5 and packet 5B step 5 — are @ts-expect-error
and type-identity assertions. Writing them exposed that they could not have meant anything.

packages/ai-tooling/tsconfig.json has include: ["src/**/*.ts"], and Vitest transpiles without
checking types. No .spec.ts file had ever been seen by a compiler. An @ts-expect-error would
have suppressed nothing and failed nothing. The plan assumed otherwise; its expected result is
"every @ts-expect-error is exercised", which requires a compiler that sees them.

Why the guard lives at the repository root

Two shapes were rejected first:

• Extending the workspace tsconfig.json — rootDir: src with composite and outDir: dist
  would emit the tests into dist, and pack:check asserts the published tarball's contents
  exactly.
• A tsconfig.test.json beside the tests — check-stage1-artifacts.mjs reports any file under
  its owned roots (packages/ai-tooling, configs/ai) that no phase manifest declares as
  undeclared-entry, and those manifests are committed records of what each phase delivered, not
  lists to be reopened.

So it is scripts/check-test-types.mjs, beside the four guards that already exist, outside the
scanner's roots.

It derives its compiler options from each workspace's own tsconfig.json rather than restating them,
minus the emit-shaped ones, so the two cannot drift apart. allowJs is the single added option and
it is load-bearing: two specs import the repository's own scripts/*.mjs guards directly, and
without it every such import is an implicit any (TS7016) and the assertions written against them
check nothing. checkJs stays off.

It is deliberately not part of pnpm check, which Stage 1 phase gates invoke literally and which
must not acquire new failure modes. It runs as its own script and its own CI step, exactly like
check:structure and check:supply-chain — inside the existing Node 24 (pnpm) job, which is
already a required check on both platforms, so no branch-protection change is needed.

Two TypeScript 7 behaviours shape the invocation, both measured rather than assumed:

diagnostic	consequence
TS5112 — command-line file arguments refused while a tsconfig.json is present	--ignoreConfig is required
TS6053 — file arguments are not glob-expanded	the file list is enumerated through git ls-files

Six real type errors, none introduced here

Turning the guard on surfaced six errors in four existing spec files: four noUncheckedIndexedAccess
violations indexing a regex capture and a Uint8Array, one unchecked sources[0], and one
exactOptionalPropertyTypes violation passing candidate: undefined explicitly where omitting the
property was meant. All six are fixed by narrowing, never by loosening a compiler option.

How it was verified

The guard fails on a real error. A deliberate const x: number = "string" added to a spec makes
it exit 1 with tests/unit/render-json.spec.ts(66,7): error TS2322; removing it restores exit 0,
and the file was confirmed byte-identical afterwards.

Every suppression is real. Removing the @ts-expect-error markers makes all five underlying
errors surface — which is the only thing that shows the assertions are load-bearing rather than
decorative:

diagnostic	what it establishes	packet
TS2558: Expected 0 type arguments, but got 1	parseStrictJson<ConfigV1> is not expressible	4B
TS2741: Property '[strictJsonBrand]' is missing	StrictJsonDocument cannot be built from a literal	4B
TS2739: 'StrictJsonDocument' is missing … from 'ConfigV1'	parser output is not a domain type	4B
TS2739: 'LockV1' is missing sources, platforms	validate('lock', …) is not ConfigV1	5B
TS2345: '"lock"' is not assignable to '"config"'	the generic follows the name, not the caller	5B

That is packet 4B step 5's three requirements and packet 5B step 5's three, and it closes the last of
the Phase 1 deferred items.

Full gate, every command exiting 0 against the committed tree:

pnpm install --frozen-lockfile --ignore-scripts
pnpm check                    # 149 unit/package/security + 7 integration = 156 tests
pnpm run check:structure      # 5 workspaces no cycle; 37 source files no boundary escape
pnpm run check:supply-chain   # 0 advisories; 0 licence conflicts
pnpm run check:test-types     # 1 workspace, 14 test sources
pnpm run changelog:check      # 0 fragments valid
node packages/ai-tooling/scripts/check-stage1-artifacts.mjs --phase 1 --tree
git diff --check

Deliberately not included

No branch-protection change. The new step runs inside the Node 24 (pnpm) job, which is already
a required check on ubuntu-latest and windows-latest; a failing step fails that job. Adding a
fourth required check would mean splitting it into its own job for no additional guarantee.

No changelog fragment. Build tooling and test corrections ship no behaviour change, and
changelog.d/ stays dormant until Stage 1 Phase 5 completes.

No product code. packages/ai-tooling/src/** is untouched; only test sources changed under that
tree.