fix: normalize CLI paths to POSIX so sourcemap sources[] is portable on Windows

[YevheniiKotyrlo]

Summary

Fixes #19. Normalizes node:path.resolve results to POSIX separators so:

1. The CLI's inputFile (which lands as sources[0] in emitted source maps) is portable per the sourcemap spec.
2. The 6 vitest CLI runner tests pass on Windows (were 1/7 before; now 7/7).
3. The 8 example vitest configs discover their test files on Windows (were "No test files found" — silently uncovered).

Same defect class onejs/one#702 standardized away from. CI is ubuntu-24.04-only, so all of this ships invisible to Windows users.

Why this matters in production

The CLI's inputFile flows into:

// packages/core/src/cli/run.ts
return {
  request: {
    filename: command.inputFile,  // ← backslash on Windows pre-fix
    source,
    sourcemap: options.sourcemap,
  },
};

That filename becomes sources[0] in the emitted source map JSON. The sourcemap spec requires forward-slash URI-style paths; Chrome DevTools, Node's source-map consumer, and most browser dev tools don't resolve backslash sources on Windows. So pre-fix, every CLI invocation on Windows emits a non-portable sourcemap.

Diff shape

File	Change
packages/core/src/cli/args.ts	Add a toPosix helper; wrap the 4 resolve(cwd, …) sites that mint inputFile/outputFile/inputSourceMapFile/sourceMapFile.
packages/core/src/cli/__tests__/run.test.ts	Replace hard-coded '/repo' with a platform-aware FAKE_CWD = process.platform === 'win32' ? 'C:/repo' : '/repo'. On Windows, node:path.resolve interprets a bare /repo as drive-relative and prepends the current drive, so the fixture has to be a fully qualified absolute path on each platform.
packages/core/__tests__/correctness.test.ts	Wrap outputsDir() / inputsDir() in toPosix.
examples/{esbuild,parcel,rolldown,rollup,rsbuild,rspack,vite,webpack}/vitest.config.ts	Inline .replaceAll('\\', '/') after resolve(import.meta.dirname, …) so tinyglobby (vitest's include matcher) actually discovers the test file on Windows.

toPosix is a one-line sep === '\\' ? p.replaceAll('\\', '/') : p — no extra dependencies, identity-on-POSIX, so this is purely additive on Linux/macOS.

Test plan

Validated on Windows MSVC + Bun 1.3.13 + Node 25.

Passes locally on this branch — relevant to the change:

• bunx vitest run src/cli/__tests__/run.test.ts — 7/7 pass (was 1/7 before)
• bunx vitest run __tests__/correctness.test.ts — 9/9 pass (still passing)
• bunx vitest run (full packages/core) — 39/39 pass (was 33/39 — 6 unblocked, 0 regressed)
• bunx vitest --config examples/esbuild/vitest.config.ts run — finds the test file (was "No test files found"). Same for the other 7 adapter examples. Test execution itself still requires bun run build on each example first; that's an unrelated test-runner ordering concern, not in this PR's scope.
• pnpm typecheck — 11/11 packages clean (turbo full)
• pnpm format:check:js — 179 files all formatted
• cargo fmt --all --check — clean (no Rust changes from this PR)

Pre-existing Windows-host limitations — out of scope, would not affect Linux CI:

These were exercised running fft's own CI matrix locally. None are introduced or affected by this PR. Linux CI (the one fft actually runs) wouldn't see these.

Check	Pre-existing failure on Windows host	Why
pnpm lint	2 errors in bench/fixtures/source.flow{,.preserve}.js	Those files are git symlinks; on Windows checkout they resolve as plain text containing the target path; oxlint parses the path string as JS syntax
pnpm codegen:rust:check	spawnSync c++ ENOENT	scripts/rust-bindings/lib.mts:83 calls c++ (Unix compiler shorthand). MSVC uses cl.exe; Clang on Windows uses clang++
pnpm build / pnpm test / pnpm e2e	turbo + napi-rs cmake-generator mismatch — Visual Studio 16 2019 cache vs Visual Studio 17 2022 env var	turbo doesn't propagate CMAKE_GENERATOR to child processes; cmake-rs falls back to oldest-installed VS
cargo test --release --workspace	hparser::tests::test1 panics: "All source locations from Hermes parser must be valid"	The MSVC EBO bug being fixed in #18. Unrelated to this PR's CLI path scope

Pre-existing lint errors on bench/fixtures/source.flow{,.preserve}.js are unrelated — they're git symlinks resolved to plain text on Windows checkout. Out of scope for this PR.

Notes

• The toPosix helper is duplicated in three files (args.ts, correctness.test.ts, and inlined in 8 vitest configs). Could be hoisted to a shared utils/ module, but each instance is one line and there's no existing utils/ shape — leaving inline keeps the diff minimal.
• The examples/*/vitest.config.ts files all share the same shape (only the test file name differs). They could be deduped via a shared helper, but each is a 1-line change to the existing config and the shape stays self-explanatory.
• This is a Windows-only behavior change. On POSIX, sep === '/' so toPosix is the identity function; the resolved path was already forward-slash. Zero impact on Linux or macOS.