fix(coverage): exempt .d.ts files from diff coverage gate

Ambient declaration files have no runtime code so v8 coverage
never generates DA records for them. Without an allowlist entry,
coverage:diff reports no-coverage-data for every .d.ts in the
diff. Add /\.d\.ts$/ to ALLOWED_GLOBS with a companion test.

Also configure @vitest/coverage-v8 for core-shared and add
targeted vitest exclusions for infrastructure files that are not
unit-testable (DI symbols, interface files, tRPC context, Sentry
SDK init) — bringing core-shared into the L2 aggregate and making
the L1 diff gate enforce coverage on new executable code.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/coverage/diff.mjs

@@ -62,6 +62,7 @@ const ALLOWED_GLOBS = [
   /\/integrations\/cms\//,
   /\/ui\//,
   // Pure type-alias / interface files (no executable code)
+  /\.d\.ts$/, // ambient declaration files — no runtime code by definition
   /\.interface\.ts$/,
   /\/index\.ts$/, // barrel re-exports — no executable code
   // Build artifacts