fix(coder): harden edit-apply pipeline against block duplication

Root cause: two proven corruption mechanisms — (M1) non-idempotent apply
stamped the same block N times when a quantized model re-emitted the same
edit_file call or a turn was retried; (M2) Levenshtein tier 4 was fail-open
with no uniqueness guard, silently splicing into the wrong location.

Fixes applied at every layer of the pipeline:

Matcher (fuzzy-match.ts): raise SIMILARITY_THRESHOLD 0.66 → 0.85; add
AMBIGUITY_EPSILON uniqueness guard — two windows within 0.05 of the top
score → ambiguous, not a guess; add block-anchor gate (≥3-line needles
require first+last line exact match before a window is scored).

Edit planner (pending_changes.ts): extract planEdit() as a pure function;
idempotency guards detect already-applied states (anchored insert re-stamp,
old-gone-but-new-present); findPendingDuplicate() collapses identical
pending rows at queue time so M1 never reaches applyOne.

Atomic writes (pending_changes.ts): temp-file + rename on the same
filesystem so a crash can't leave a half-written source file; realpath()
first so symlinks survive the rename.

Per-file mutex (pending_changes.ts): withFileLock() serializes concurrent
read-modify-write on the same path via a chained-Promise Map.

EOL preservation (pending_changes.ts): normalize CRLF → LF for matching,
restore native line ending on write so Windows-style files stay clean.

Context isolation (inference_context.ts): replace module-level singleton
with AsyncLocalStorage so concurrent inference runs (arena parallel
dispatch, dispatcher poll racing a user message) each get their own
scoped context with no clobbering.

Tests: plan-edit.test.ts (pure planEdit unit tests), extended fuzzy-match
and pending_changes_integration suites, ALS isolation test that proves
overlapping runs get correct session IDs.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/coder/src/services/__tests__/fuzzy-match.test.ts

@@ -161,6 +161,52 @@ describe('locateMatch — strategy 4: Levenshtein', () => {
   });
 });
 
+describe('locateMatch — strategy 4: fail-closed on ambiguity (corruption guard)', () => {
+  it('refuses (ambiguous) when two equally-similar anchored blocks both clear the bar', () => {
+    // The repetitive-file case that duplicated blocks: two blocks share the same
+    // first+last anchor lines and their middle lines are EQUALLY similar to the
+    // (drifted) needle. Tier 4 must refuse rather than splice over one of them.
+    const content = [
+      'const x = {',
+      '  total = aa;',
+      '};',
+      'const x = {',
+      '  total = bb;',
+      '};',
+    ].join('\n');
+    const needle = ['const x = {', '  total = ab;', '};'].join('\n');
+    const result = locateMatch(content, needle);
+    expect(result.kind).toBe('ambiguous');
+  });
+
+  it('refuses a below-threshold near-miss that the old 0.66 floor would have spliced', () => {
+    // ~0.7 similar: under the raised 0.85 floor this is now not_found, so the
+    // caller surfaces a correctable error instead of corrupting the file.
+    const content = 'const grandTotalAmount = a + b;\n';
+    const needle = 'const totalValue = a + b;';
+    const result = locateMatch(content, needle);
+    expect(result).toEqual({ kind: 'not_found' });
+  });
+
+  it('still matches a single genuine high-similarity drift uniquely', () => {
+    const content = 'const total = sum + tax;\n';
+    const needle = 'const totals = sum + tax;'; // one-char typo, ~0.96
+    const result = locateMatch(content, needle);
+    expect(result.kind).toBe('fuzzy');
+    const { start, end } = span(result);
+    expect(content.slice(start, end)).toBe('const total = sum + tax;');
+  });
+
+  it('requires an exact first+last line anchor for multi-line needles', () => {
+    // First line drifted too far to anchor → no window is scored → not_found,
+    // even though the middle lines are identical.
+    const content = ['function compute() {', '  return a + b;', '  return done;', '}'].join('\n');
+    const needle = ['totally different opener', '  return a + b;', '}'].join('\n');
+    const result = locateMatch(content, needle);
+    expect(result).toEqual({ kind: 'not_found' });
+  });
+});
+
 describe('locateMatch — edge cases', () => {
   it('returns not_found for an empty needle', () => {
     expect(locateMatch('anything', '')).toEqual({ kind: 'not_found' });