fix(producer): scope per-instance variables for repeated sub-composition mounts (#2070)

#2066 fixed sub-composition data-variable-values on the render path for a single
mount, but the reusable-template pattern from #2064 (the same sub-comp mounted
multiple times with different values) still diverged from preview/snapshot:
every mount shared one __hfVariablesByComp key and one CSS scope selector, so
the last mount's values clobbered the earlier ones and all-but-one instance
rendered blank.

The producer now assigns per-instance runtime composition ids
(assignBundledRuntimeCompositionIds) and threads hostIdentityMap into the shared
inliner, mirroring the preview bundler. The shared inliner's default
buildScopeSelector already scopes by the runtime id, and timelines remap to it
via the scoping proxy, so each instance's variables, CSS, and timeline land
under its own id.

Pixel-verified end to end: two mounts of one sub-comp with different
data-variable-values now render their own content (green CARD_A / blue CARD_B),
matching snapshot; single-instance behavior is unchanged.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/core/src/compiler/htmlBundler.ts

@@ -305,7 +305,7 @@ function uniqueCompositionId(baseId: string, index: number): string {
   return `${baseId}__hf${index}`;
 }
 
-type BundledHostCompositionIdentity = {
+export type BundledHostCompositionIdentity = {
   authoredCompositionId: string | null;
   runtimeCompositionId: string | null;
 };
@@ -351,7 +351,8 @@ function countBundledAuthoredCompositionIds(hosts: Element[]): Map<string, numbe
   return counts;
 }
 
-function assignBundledRuntimeCompositionIds(
+// fallow-ignore-next-line complexity
+export function assignBundledRuntimeCompositionIds(
   hosts: Element[],
   counts: Map<string, number> = countBundledAuthoredCompositionIds(hosts),
 ): Map<Element, BundledHostCompositionIdentity> {

packages/core/src/compiler/index.ts

@@ -28,6 +28,8 @@ export { compileHtml, type MediaDurationProber } from "./htmlCompiler";
 
 // HTML bundler (Node.js — requires fs, linkedom, esbuild)
 export {
+  assignBundledRuntimeCompositionIds,
+  type BundledHostCompositionIdentity,
   bundleToSingleHtml,
   type BundleOptions,
   prepareFlattenedInnerRoot,