Consolidate duplicated kb test helpers into a shared test-support module

[williamthorsen]

Problem

Every test file in @codeassembly/kb re-rolls its own copies of trivial filesystem and KB-scaffolding test helpers: temp-directory creation, path-existence checks, and KB-root / store / registry-path builders. Several are verbatim duplicates (for example makeTempDir, a local exists wrapper, and makeRegistryPath); others are near-verbatim variants of the same idea. There is no shared test-support home, so each new test file re-rolls these helpers.

Context

• The duplication is uniform across the package: both pre-existing tests and the tests added in Add a kb create command to provision new KB stores #720 follow this per-file pattern, so it reads as an established convention rather than a one-off oversight.
• Tests run directly from TypeScript source through Vitest with no build step, and already import sibling .ts modules with honest .ts specifiers. A shared helper is imported the same way; there is no compile barrier for tests.
• The only build consideration is keeping the shared module out of the published dist. nmr compile (the shared config/build.ts) emits every src/**/*.ts file except those matched by its ignore globs, and it is glob-driven rather than tsconfig-driven, so a per-package tsconfig exclude cannot affect it. The chosen mechanism is to add a test-utils directory name to the shared build's ignore list.
• The project keeps test files as per-directory __tests__/ siblings of the modules they test and avoids a rolled-up src/__tests__/. A package-wide shared module therefore gets its own purpose-named home rather than nesting in the package-root test directory.
• Scope is the kb package only for the consolidation itself. The single shared-config touch (the test-utils ignore entry) is additive and inert for packages that have no such directory.

Proposed solution

Introduce one kb-local test-support module under a new src/test-utils/ directory, and exclude that directory from the published build by adding it to the shared build's ignore globs. Migrate the kb test files to import the genuinely shared primitives (temp-directory creation and the KB-root / store / registry builders) from this module, consolidating each to a single copy. Replace the re-rolled local exists wrappers with the existing production pathExists / directoryExists from the filesystem module rather than duplicating them at all. Leave per-test setup that genuinely varies inside the individual tests.

The new module should itself model the repo's code conventions, since it is a module of helper functions: functions in case-insensitive alphabetical order absent a more compelling ordering, any internal helpers at the end, and a concise description on every exported function.

Acceptance criteria

Must have

• A single kb-local test-support module under src/test-utils/ provides the shared filesystem and KB-root / store / registry test helpers.
• The module is excluded from the published build (dist) by adding test-utils to the shared build's ignore globs; no tsconfig plumbing or per-package build workaround is introduced.
• kb test files import the shared helpers instead of redefining them; genuinely per-test setup stays local to each test.
• Re-rolled local exists wrappers are replaced by the production pathExists / directoryExists rather than moved into the shared module.
• No rolled-up src/__tests__/ is introduced.
• The shared module follows the repo code conventions: functions alphabetized absent a more compelling order, helpers at the end, and a concise description on every function.
• The kb test suite stays green.

Dependencies

• Relates to Add a kb create command to provision new KB stores #720, whose new test files follow the existing per-file duplication convention this consolidates.