perf: cap/batch untracked-file diff subprocess fan-out

[uditdc]

Closes #10

GET /diff/working previously spawned one git diff --no-index subprocess per untracked file via Promise.all, so a repo with hundreds of untracked files (e.g. an accidental node_modules add) would fan out hundreds of concurrent processes.

This adds a small mapWithConcurrency helper in src/server/routes/diff.ts and uses it to cap the untracked-file diffing to 8 concurrent subprocesses instead of adding a new dependency for it.

Acceptance criteria

• Add a concurrency cap or batch into fewer invocations — added mapWithConcurrency with a cap of 8 concurrent git diff --no-index calls in diffRoutes's /diff/working handler.
• Verify manually or via a fixture with 200+ untracked files that process count stays bounded — tests/concurrency.test.ts exercises mapWithConcurrency with a 250-item fixture and asserts the number of in-flight calls never exceeds the configured limit (also covers full/in-order processing and edge cases like an empty list).

pnpm typecheck, pnpm test, and pnpm build all pass.

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