perf(variables): O(N+V) map lookups in template resolve + header pass

Address gemini's perf findings on PR #92:

- variableBinding.applyBindingToObject: build a name→Variable map
  once per call so a field with N markers resolves in O(N+V)
  instead of N linear scans across the V Variables.
- zplGenerator.planTemplateHeader: build id/name/fn maps once
  before the leaf walk so the per-leaf single-bind lookup +
  per-marker name lookup + per-fn header emit all stay constant-
  time. Cuts the worst case from O(N·V) to O(N+V).

Per-field Map rebuild inside zplParser.applyFnEmbeds (gemini's
4th finding) deferred — that path also bootstraps Variables so a
hoisted-cache invalidation strategy adds more complexity than the
inner-loop scan saves at current scale.

TemplateContentInput a11y findings (role=menuitem, keyboard nav)
deferred to a separate a11y pass alongside other Properties
components — keeping this PR scoped to the ^FE feature surface.

Author: u8array

src/lib/variableBinding.ts

@@ -188,10 +188,11 @@ export function applyBindingToObject<T extends LabelObject>(
     next = resolveVariableValue(variable, active, mode);
   }
   if (hasTemplateMarkers(next)) {
+    // Map lookup so a field with N markers stays O(N+V) not O(N·V).
+    const byName = new Map(variables.map((v) => [v.name, v]));
     next = resolveTemplateMarkers(next, (name) => {
-      const v = variables.find((x) => x.name === name);
-      if (!v) return undefined;
-      return resolveVariableValue(v, active, mode);
+      const v = byName.get(name);
+      return v ? resolveVariableValue(v, active, mode) : undefined;
     });
   }
   if (next === content) return obj;

src/lib/zplGenerator.ts

@@ -87,20 +87,28 @@ function planTemplateHeader(
   label: LabelConfig,
   variables: readonly Variable[],
 ): { headerLines: string[]; emitCtx: ZplEmitContext } {
+  // Pre-built maps keep the leaf walk + header emit O(N+V) instead
+  // of O(N·V) — a label with hundreds of objects and dozens of
+  // variables would otherwise re-scan the whole variables list per
+  // leaf per marker.
+  const varsById = new Map(variables.map((v) => [v.id, v]));
+  const varsByName = new Map(variables.map((v) => [v.name, v]));
+  const varsByFn = new Map(variables.map((v) => [v.fnNumber, v]));
+
   const templatePayloads: string[] = [];
   const templateFns = new Set<number>();
   const singleBindFns = new Set<number>();
   for (const leaf of flattenObjects(shifted)) {
     if (leaf.includeInExport === false) continue;
     if (leaf.variableId) {
-      const v = variables.find((x) => x.id === leaf.variableId);
+      const v = varsById.get(leaf.variableId);
       if (v) singleBindFns.add(v.fnNumber);
     }
     const c = getObjectStringContent(leaf);
     if (c === undefined || !hasTemplateMarkers(c)) continue;
     templatePayloads.push(c);
     for (const name of extractTemplateRefs(c)) {
-      const v = variables.find((x) => x.name === name);
+      const v = varsByName.get(name);
       if (v) templateFns.add(v.fnNumber);
     }
   }
@@ -114,7 +122,7 @@ function planTemplateHeader(
   if (pickedEmbedChar !== '#') headerLines.push(`^FE${pickedEmbedChar}`);
   for (const fn of [...templateFns].sort((a, b) => a - b)) {
     if (singleBindFns.has(fn)) continue;
-    const v = variables.find((x) => x.fnNumber === fn);
+    const v = varsByFn.get(fn);
     if (!v) continue;
     headerLines.push(`^FN${fn}${fdField(v.defaultValue)}`);
   }