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/tools/create_file.ts

@@ -1,6 +1,7 @@
 import { z } from 'zod';
 import type { ToolDef, ToolContext } from './types.js';
 import { queueCreate } from '../pending_changes.js';
+import { denyReadOnly, finalizeWrite } from './write-gate.js';
 
 const CreateFileInput = z.object({
   file_path: z.string().min(1),
@@ -32,6 +33,7 @@ export const createFileTool: ToolDef<CreateFileInputT> = {
     },
   },
   async execute(input: CreateFileInputT, projectRoot: string, context: ToolContext): Promise<unknown> {
+    if (context.permissionMode === 'plan') return denyReadOnly('create_file');
     const change = await queueCreate(
       context.sql,
       context.sessionId,
@@ -40,12 +42,11 @@ export const createFileTool: ToolDef<CreateFileInputT> = {
       input.content,
       projectRoot,
     );
-    return {
-      status: 'queued',
-      change_id: change.id,
-      file_path: change.file_path,
-      operation: 'create',
-      message: `File creation queued: ${change.file_path}. Use apply_pending to write changes to disk.`,
-    };
+    return finalizeWrite(
+      context,
+      projectRoot,
+      change,
+      `File creation queued: ${change.file_path}. Use apply_pending to write changes to disk.`,
+    );
   },
 };