feat(history): add visible-parent resolver and status optimizations

Introduce an iterative visibleParentResolver for commit-graph rendering,
replacing a recursive traversal. It preserves first-parent ordering,
deduplicates converging paths, and avoids stack overflows; a test suite
validates correctness and performance. Additionally, streamline background
fetch handling in the UI to reuse fetchRemote's returned status and avoid
unnecessary status reads and tab invalidation, and refactor Git status
handling in Rust to build a file_status_index that preserves rename and
first-match semantics while speeding lookups.

- Add visibleParentResolver + comprehensive tests for ancestry handling
- Reuse fetchRemote result to apply status and skip unchanged updates
- Replace status_for_file with file_status_index to improve performance

scripts/graph-parents.test.mjs

@@ -0,0 +1,51 @@
+import assert from 'node:assert/strict';
+import { readFileSync } from 'node:fs';
+import test from 'node:test';
+import ts from 'typescript';
+const source = readFileSync(new URL('../src/lib/graphParents.ts', import.meta.url), 'utf8');
+const compiled = ts.transpileModule(source, { compilerOptions: { module: ts.ModuleKind.ESNext, target: ts.ScriptTarget.ES2022 } }).outputText;
+const { visibleParentResolver } = await import(`data:text/javascript;base64,${Buffer.from(compiled).toString('base64')}`);
+function oldResolver(hash, visible, commits, seen = new Set()) {
+  if (visible.has(hash)) return [hash];
+  if (seen.has(hash)) return [];
+  seen.add(hash);
+  const commit = commits.get(hash);
+  return commit ? [...new Set(commit.parents.flatMap(parent => oldResolver(parent, visible, commits, new Set(seen))))] : [];
+}
+test('preserves parent order and deduplicates converging paths', () => {
+  const items = [{hash:'tip',parents:['a','b']},{hash:'a',parents:['x','y']},{hash:'b',parents:['y','z']}];
+  const resolve = visibleParentResolver(items,new Set(['x','y','z']));
+  assert.deepEqual(resolve('tip'),['x','y','z']);
+  assert.deepEqual(resolve('missing'),[]);
+  assert.deepEqual(resolve('x'),['x']);
+});
+test('matches previous traversal across deterministic merge DAGs and visibility filters', () => {
+  let seed=42;
+  const random=()=>((seed=(Math.imul(seed,1664525)+1013904223)>>>0)/2**32);
+  for(let run=0;run<80;run++) {
+    const items=Array.from({length:40},(_,i)=>({hash:String(i),parents:i===39?[]:[String(i+1),...(random()<.5?[String(i+1+Math.floor(random()*(39-i)))]:[])]}));
+    const visible=new Set(items.filter(()=>random()<.35).map(x=>x.hash));
+    const resolve=visibleParentResolver(items,visible);
+    const map=new Map(items.map(x=>[x.hash,x]));
+    for(const item of items) assert.deepEqual(resolve(item.hash),oldResolver(item.hash,visible,map));
+  }
+});
+test('handles 20000 hidden ancestors without overflowing the call stack', () => {
+  const items=Array.from({length:20000},(_,i)=>({hash:String(i),parents:[String(i+1)]}));
+  assert.deepEqual(visibleParentResolver(items,new Set(['20000']))('0'),['20000']);
+});
+test('shared merge ancestry is expanded only once', () => {
+  let reads=0;
+  const items=Array.from({length:30},(_,i)=>({hash:String(i),get parents(){reads++;return i===29?['root']:[String(i+1),String(Math.min(i+2,29))];}}));
+  const resolve=visibleParentResolver(items,new Set(['root']));
+  assert.deepEqual(resolve('0'),['root']);
+  const initial=reads;
+  assert.deepEqual(resolve('1'),['root']);
+  assert.equal(reads,initial);
+  assert.ok(reads<300);
+});
+const items=Array.from({length:24},(_,i)=>({hash:String(i),parents:i===23?['root']:[String(i+1),String(Math.min(i+2,23))]}));
+const visible=new Set(['root']);const map=new Map(items.map(x=>[x.hash,x]));
+const before=performance.now();oldResolver('0',visible,map);const oldMs=performance.now()-before;
+const after=performance.now();visibleParentResolver(items,visible)('0');const newMs=performance.now()-after;
+console.log(`Synthetic shared-ancestry benchmark (24 commits): old ${oldMs.toFixed(2)} ms, new ${newMs.toFixed(2)} ms`);