perf: async main-process file I/O and debounced log counter

[oahsiao]

Summary

Performance improvements with no behavior or API changes — purely internal. Three hotspots are addressed: blocking file I/O in the main process and an unthrottled counter in the renderer.

Changes

1. Non-blocking file read — fs:readText (src/main/ipc.js)

The viewer's file reader used synchronous fs.statSync / fs.openSync / fs.readSync / fs.closeSync, reading up to 5 MB on the main thread. While a large LOG loads, the main process event loop is frozen, so every window and all IPC stall.

• Converted to fs.promises (fsp.stat, fsp.open, FileHandle.read, FileHandle.close).
• The 5 MB cap, binary-detection (NUL probe), truncated flag and the returned object shape are all unchanged.

2. Non-blocking directory listing — fs:list (src/main/ipc.js)

The file-tree listing did a blocking fs.statSync per entry, serializing one syscall after another on large folders.

• Converted to async fsp.readdir + fsp.stat.
• Entry stats now run concurrently via Promise.all, so listing a big folder no longer blocks the main process and finishes faster.
• Per-entry stat failures are still ignored individually; return shape unchanged.

3. Debounced LOG counter — src/renderer/js/app.js

The line/character counter ran on every input event of the LOG textarea, re-splitting the entire buffer with a regex on each keystroke — noticeably laggy when editing or pasting very large logs.

• Added updateCounterSoon() (150 ms debounce) and wired it to the textarea input handler.
• Context switches (tab change, language switch, initial render) still call updateCounter() immediately, so the displayed count is always correct when the active LOG changes.

Why

• Keeps the UI responsive while opening large logs and browsing big output folders.
• Eliminates per-keystroke work proportional to log size in the editor.

Compatibility / risk

• Low risk. IPC return shapes are identical, so the renderer needs no changes.
• No new dependencies; no public API or persisted-data changes.

Validation

• Verified with the editor language service — no errors in either file.
• The existing static self-test suite (test/selftest.test.js) covers i18n keys, highlight regex compilation and duplicate ids; it does not touch these code paths, so its behavior is unaffected. Please run npm test in CI / a Node environment to confirm (Node was not available in the dev environment used here).

Files changed

• src/main/ipc.js — async fs:readText and fs:list
• src/renderer/js/app.js — debounced counter