fix(build): resolve tsdown vendor alias with fileURLToPath

[kkkhs]

Summary

Closes #3353.

The tsdown alias for @limrun/xdelta3-wasm now resolves the fixture URL with fileURLToPath() instead of URL.pathname, so file:///C:/... becomes a Windows drive-letter path rather than /C:/.... Touched files: 1.

Validation

Tested on 12e626979.

base 542fb77b3 expression	head 12e626979 expression
URL.pathname -> /C:/repo/src/vendor/limrun-delta-sync-omitted.ts	fileURLToPath(..., { windows: true }) -> C:\repo\src\vendor\limrun-delta-sync-omitted.ts

Passed: pnpm format:check, pnpm lint, pnpm typecheck, pnpm build, pnpm check:bundle-owner-files, pnpm check:package, pnpm check:mcp-metadata, pnpm check:gate-manifest, pnpm check:gate-manifest:test, git diff --check.

pnpm check:affected --run reached Fallow and stopped because this host's glibc 2.28 is below the bundled Fallow binary's GLIBC_2.29/2.30 requirement; earlier runnable gates passed. Not run on native Windows here.

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[cubic-dev-ai]

No issues found across 1 file

_{View guided diff | Re-trigger cubic}

[thymikee]

This PR is ready for review at 12e6269. The tsdown alias now uses fileURLToPath, and I found no problems in the change. All checks pass, and the build job exercises the changed alias path. There are no conflicts. I did not run the Windows build locally.

Not blocking: scripts/fuzz/envelope.ts:112 and one test still use the new URL(import.meta.url).pathname pattern. You can switch those two sites to fileURLToPath in a follow-up, or leave them.