perf(camera): remove redundant mkdir subprocess

[JustYannicc]

What changed

• Removed the redundant /bin/mkdir -p subprocess from camera capture before writing PNG bytes.
• Added a focused camera regression script that asserts capture no longer invokes /bin/mkdir, preserves the capture save path shape, writes through fsWriteBinaryFile(savePath, bytes), and verifies the main IPC writer creates parent directories recursively before writing.

Why

• fs-write-binary-file already owns recursive parent directory creation with fs.promises.mkdir(path.dirname(filePath), { recursive: true }).
• Avoiding the extra subprocess removes the measured directory-step overhead: /bin/mkdir -p averaged 1.370 ms over 80 runs on the audit Mac, while in-process mkdir averaged 0.033 ms.

Compatibility impact

• Camera captures keep the same Pictures/SuperCmd Captures/supercmd-capture-*.png path behavior.
• Save failures still fall through the existing capture error handling.
• No new renderer-to-main bridge or IPC contract changes.

How tested

• node --test scripts/test-camera-capture-single-encode.mjs
• mcp__lsp.diagnostics on src/renderer/src/CameraExtension.tsx was run; upstream main currently reports the existing setCapturePreviewClearTimerRef diagnostic unrelated to this diff.
• ./node_modules/.bin/tsc -p tsconfig.renderer.json --noEmit was attempted; upstream main has existing renderer type errors unrelated to this diff.
• git diff --check origin/main..HEAD

Stack validation

• Clean PR branch created from origin/main (9bdd3d2) and contains only this camera mkdir fix commit (352b8ed).
• The implementation was first validated on the integration-stack branch from 52288a0, then reapplied to this clean upstream-main PR branch.
• This PR does not include integration-stack commits and does not require perf(runtime): reduce extension and menubar churn #530-perf(search): reduce indexing and ranking overhead #535 for the isolated mkdir removal.

[JustYannicc]

Superseded by #556, which consolidates this AI/native media performance batch.