Live: stop the preflight writing source, cache the resolution

The polling-rework preflight wrote the variant scaffold into source during the
poll lease, before the agent acted. On source-preview targets (React/Vue/Vite,
everything but the svelte-component path) that write full-reloaded the
framework; a browser caught mid-reload missed the agent's variant write and the
SSE done, and sat stranded at 0/N.

Restore the 3.5 single-atomic-edit semantics: the preflight still resolves the
element location and computes the scaffold, but --defer-source-write leaves
source untouched and hands the agent the wrapper text plus the picked source
range. The agent splices variants into the wrapper and replaces the range in
one write, so the framework reloads exactly once. The svelte-component path is
untouched (it never writes route source). The missed-completion recovery stays
as defense in depth.

Also cache the resolved source file per target signature (locator + route):
the ~7.6s tree search re-ran on every generate for the same element; a hit now
points the helper straight at the file via --file, invalidated when the target
changes or a resolution fails.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

skill/scripts/live/generation-preflight.mjs

@@ -5,7 +5,32 @@ import { promisify } from 'node:util';
 const execFileAsync = promisify(execFile);
 const PREFLIGHT_TIMEOUT_MS = 15_000;
 
-export function buildGenerationPreflight(event, scriptsDir) {
+// Per-target cache of the resolved source file. The wrap search walks the whole
+// project tree and was measured at ~7.6s on a large repo; it re-ran on every
+// generate for the same picked element (re-rolls, param passes). Keyed by the
+// target signature (locator + route), so it invalidates automatically when the
+// element or route changes; a failed resolution evicts its entry (see below).
+const sourceResolutionCache = new Map();
+
+/** Test/lifecycle hook: drop all cached source resolutions. */
+export function clearSourceResolutionCache() {
+  sourceResolutionCache.clear();
+}
+
+function targetSignature(event) {
+  const isInsert = event.mode === 'insert';
+  const target = isInsert ? insertTarget(event) : replaceTarget(event);
+  return JSON.stringify({
+    mode: isInsert ? 'insert' : 'replace',
+    position: isInsert ? target.position : null,
+    elementId: target.elementId || null,
+    classes: target.classes || null,
+    tag: target.tag || null,
+    pageUrl: event.pageUrl || null,
+  });
+}
+
+export function buildGenerationPreflight(event, scriptsDir, { cache = null } = {}) {
   if (!event || event.type !== 'generate' || !event.id) return null;
 
   const isInsert = event.mode === 'insert';
@@ -14,13 +39,24 @@ export function buildGenerationPreflight(event, scriptsDir) {
 
   const script = path.join(scriptsDir, isInsert ? 'live-insert.mjs' : 'live-wrap.mjs');
   const args = [script, '--id', event.id, '--count', String(event.count || 3)];
+  // Compute the scaffold but do not write it into source for source-preview
+  // targets. The agent writes wrapper + variants atomically; a premature
+  // server-side write reloads the framework and strands the browser at 0/N.
+  // No-op on the svelte-component path, which never writes the route source.
+  args.push('--defer-source-write');
   if (isInsert) args.push('--position', target.position);
   if (target.elementId) args.push('--element-id', target.elementId);
   if (target.classes) args.push('--classes', target.classes);
   if (target.tag) args.push('--tag', target.tag);
   if (target.text) args.push('--text', target.text);
   if (!isInsert && event.pageUrl) args.push('--page-url', event.pageUrl);
-  return { script, args, mode: isInsert ? 'insert' : 'replace' };
+  const signature = targetSignature(event);
+  // A cached resolution points the helper straight at the file, skipping the
+  // tree search. The helper still reads current content, so line ranges stay
+  // fresh; only discovery is cached.
+  const cachedFile = cache ? cache.get(signature) : null;
+  if (cachedFile) args.push('--file', cachedFile);
+  return { script, args, mode: isInsert ? 'insert' : 'replace', signature };
 }
 
 /**
@@ -38,8 +74,9 @@ export async function runGenerationPreflight(event, {
   scriptsDir,
   execFileImpl = execFileAsync,
   timeoutMs = PREFLIGHT_TIMEOUT_MS,
+  cache = sourceResolutionCache,
 } = {}) {
-  const command = buildGenerationPreflight(event, scriptsDir);
+  const command = buildGenerationPreflight(event, scriptsDir, { cache });
   if (!command) {
     return { ok: false, skipped: true, reason: 'insufficient_locator' };
   }
@@ -53,13 +90,23 @@ export async function runGenerationPreflight(event, {
     });
     const line = String(stdout).trim().split('\n').filter(Boolean).pop();
     if (!line) throw new Error('preflight returned no scaffold metadata');
+    const scaffold = JSON.parse(line);
+    // Cache the resolved SOURCE file (route source, not the svelte manifest) so
+    // the next generate on this target skips the tree search.
+    const resolvedSource = scaffold.sourceFile || scaffold.file;
+    if (cache && command.signature && typeof resolvedSource === 'string') {
+      cache.set(command.signature, resolvedSource);
+    }
     return {
       ok: true,
       mode: command.mode,
       durationMs: performance.now() - startedAt,
-      scaffold: JSON.parse(line),
+      scaffold,
     };
   } catch (error) {
+    // Evict a stale/failed resolution so the next attempt does a full search
+    // (the element may have moved out of the previously cached file).
+    if (cache && command.signature) cache.delete(command.signature);
     return {
       ok: false,
       mode: command.mode,