Summary

Preserve original file encoding (GB18030, UTF-8 BOM) on the read/edit/write/snapshot/restore path. Closes #1445.

The bug chain reported in #1445

User on Chinese Windows 11, YOLO mode:

1. read_file / edit_file return mangled bytes or fail SEARCH on GB18030 files (most CN Windows projects)
2. Model falls back to PowerShell — same encoding issue, also fails
3. Model's last resort: git show HEAD:<file> + replay edits from conversation history
4. That last resort is token-expensive AND lossy — any user manual edits between the start of the turn and the failure are silently overwritten

Root cause is layer 1. The other layers are the model improvising around the silent corruption.

Why this happened

Every filesystem call in the edit pipeline hard-coded UTF-8:

• src/tools/filesystem.ts:283 — read_file: raw.toString("utf8")
• src/tools/filesystem.ts:656 — write_file: writeFile(abs, content, "utf8")
• src/tools/fs/edit.ts:16,32 — applyEdit read+write
• src/tools/fs/edit.ts:86,126,132 — applyMultiEdit read, write, rollback-write
• src/code/edit-blocks.ts:160,187,213,243,275 — applyEditBlock content, applyEditBlock write, toWholeFileEditBlock, snapshotBeforeEdits, restoreSnapshots

The shell stdout path had already solved the equivalent problem: smartDecodeOutput at src/tools/shell/exec.ts:177 does UTF-8-fatal → GB18030 fallback on Windows. The filesystem path just never used the same approach.

What this PR does

New src/code/file-encoding.ts with two pure helpers:

• decodeFileBuffer(buf) → { text, encoding: 'utf8' | 'utf8-bom' | 'gb18030' }
  • UTF-8 BOM detected by leading EF BB BF, stripped from returned text
  • UTF-8 attempted in fatal mode; falls through to GB18030 fallback on Windows
  • Last-resort lossy UTF-8 with replacement chars (matches smartDecodeOutput's safety net so we never throw on read)
• encodeFile(text, encoding) → Buffer
  • UTF-8 / UTF-8-BOM via Buffer.from
  • GB18030 via iconv-lite (new dep — 0-dep, MIT, ~50KB, 30M weekly downloads)

All 12 call sites above swap to these helpers. The edit pipeline now:

1. Reads bytes → decodes via decodeFileBuffer → text string + detected encoding
2. Applies SEARCH/REPLACE on the text (existing logic unchanged)
3. Writes via encodeFile(after, encoding) so the file keeps its original encoding

EditSnapshot gains a prevEncoding?: FileEncoding field so restoreSnapshots round-trips correctly when undoing edits on GB18030 / BOM files. Field is optional for backward compat with any persisted older snapshots — restore defaults to UTF-8 when absent.

What this PR does NOT do

• Doesn't change line-ending handling (CRLF/LF detection at lineEndingOf stays the same — orthogonal to encoding).
• Doesn't convert files. A GB18030 file stays GB18030. Any user expecting "Reasonix should normalize all files to UTF-8" is out of scope; that's a destructive workflow we shouldn't ship silently.
• Doesn't detect arbitrary encodings (UTF-16, Big5, Shift-JIS, etc.). UTF-8 and GB18030 cover the actual reported failures; adding more without real demand is over-fitting. If those hit later, the helper is the right place to grow.
• Doesn't address the secondary request in read_file、edit_file失败导致一轮对话中的修改丢失 #1445 ("per-prompt undo of all changes including user manual edits"). The existing checkpoint infrastructure in src/code/checkpoints.ts covers that surface; a per-turn auto-snapshot hook is a separate PR if desired. With the root encoding bug fixed, edit_file shouldn't fail in the first place, which removes the trigger for the entire model-fallback chain the user described.

Test Plan

• tests/file-encoding.test.ts — 8 cases covering plain UTF-8, multi-byte UTF-8, BOM strip + report, GB18030 fallback (Windows-only), empty file, round-trip in each encoding
• tests/edit-blocks.test.ts — 3 new cases: edit GB18030 file stays GB18030 (asserts UTF-8 decode fails on the result); edit BOM file preserves the BOM bytes; snapshot+restore round-trips through GB18030
• npx vitest run tests/file-encoding.test.ts tests/edit-blocks.test.ts tests/filesystem-tools.test.ts tests/comment-policy.test.ts — 168 pass
• npx tsc --noEmit — clean
• npx biome check on touched files — clean
• npm run verify via pre-push hook — green