fix(producer): rewrite relative asset paths when inlining sub-compositions (#166)

## Summary

- **Fixes**: `<img src="../icon.svg">` and similar `../` relative asset references in sub-compositions resolve to 404 after inlining into the root document
- **Unifies**: Both `hyperframes preview` (core bundler) and `hyperframes render` (producer) now use the same shared logic — no duplication

## Root cause

When inlining a sub-composition at `compositions/scene.html` into root `index.html`, a relative path like `../icon.svg` is correct from `compositions/` (it points to project root) but after inlining, `../` escapes the project directory.

## Fix

Extracts path rewriting into a shared `rewriteSubCompPaths.ts` utility in `@hyperframes/core`, used by both the bundler and the producer. **Only rewrites paths starting with** **`../`** — plain relative paths like `assets/foo.svg` are already correct from the root perspective and must not be rewritten (this was the regression cause in `overlay-montage-prod`: sub-composition asset refs like `assets/notch.svg` were incorrectly being rewritten to `compositions/assets/notch.svg`).

## Regression fix

The earlier version of this fix (now in history) rewrote ALL relative paths including `assets/foo.svg`, breaking the `overlay-montage-prod` regression test. This PR fixes that by scoping rewrites to `../`\-prefixed paths only.

## Test plan

- [x] `../icon.svg` in sub-composition renders correctly in both preview and render
- [x] `assets/foo.svg` (no `../`) in sub-composition still resolves correctly — not rewritten
- [x] `overlay-montage-prod` regression test passes
- [x] All other regression shards pass

packages/core/src/compiler/htmlBundler.ts

@@ -3,6 +3,7 @@ import { join, resolve, isAbsolute, sep } from "path";
 import * as cheerio from "cheerio";
 import { transformSync } from "esbuild";
 import { compileHtml, type MediaDurationProber } from "./htmlCompiler";
+import { rewriteAssetPaths } from "./rewriteSubCompPaths";
 import { validateHyperframeHtmlContract } from "./staticGuard";
 
 /** Resolve a relative path within projectDir, rejecting traversal outside it. */
@@ -432,6 +433,20 @@ export async function bundleToSingleHtml(
       $content(s).remove();
     });
 
+    // Rewrite relative asset paths before inlining so ../foo.svg from
+    // compositions/ resolves correctly when the content moves to root.
+    const $assetEls = $innerRoot.length
+      ? $innerRoot.find("[src], [href]")
+      : $content("[src], [href]");
+    rewriteAssetPaths(
+      $assetEls.toArray(),
+      src,
+      (el, attr) => $content(el).attr(attr),
+      (el, attr, val) => {
+        $content(el).attr(attr, val);
+      },
+    );
+
     if ($innerRoot.length) {
       const innerCompId = $innerRoot.attr("data-composition-id");
       const innerW = $innerRoot.attr("data-width");

packages/core/src/compiler/rewriteSubCompPaths.ts

@@ -0,0 +1,92 @@
+/**
+ * Rewrite relative asset paths in sub-composition content so they resolve
+ * correctly after the content is inlined into the root document.
+ *
+ * A sub-composition at "compositions/scene.html" referencing "../icon.svg"
+ * means the project root — but after inlining into root index.html, the
+ * "../" escapes the project directory and causes 404s. This function
+ * resolves each relative path against the sub-composition's directory,
+ * then normalizes it to be relative to the project root.
+ *
+ * Used by both the core bundler (preview) and the producer compiler (render)
+ * to ensure consistent behavior.
+ */
+
+import { join, resolve, dirname } from "path";
+
+/** Attributes that may contain relative asset paths. */
+const PATH_ATTRS = ["src", "href"] as const;
+
+/** Protocols and prefixes that should never be rewritten. */
+function isAbsoluteOrSpecial(val: string): boolean {
+  return (
+    !val ||
+    val.startsWith("http://") ||
+    val.startsWith("https://") ||
+    val.startsWith("//") ||
+    val.startsWith("data:") ||
+    val.startsWith("#")
+  );
+}
+
+/**
+ * Returns true only for paths that traverse up with `../`.
+ * Plain relative paths like `assets/foo.svg` are already correct from the
+ * root perspective — the browser resolves them against the served root, which
+ * is the project root, so they don't need rewriting.
+ */
+function needsRewrite(val: string): boolean {
+  return val.startsWith("../") || val === "..";
+}
+
+/**
+ * Rewrite a single relative path from a sub-composition's context to the
+ * project root context.
+ *
+ * @param compSrcPath - The `data-composition-src` value (e.g. "compositions/scene.html")
+ * @param relativePath - The asset path to rewrite (e.g. "../icon.svg")
+ * @returns The rewritten path relative to project root (e.g. "icon.svg"), or
+ *          the original path if no rewriting is needed.
+ */
+export function rewriteAssetPath(compSrcPath: string, relativePath: string): string {
+  if (isAbsoluteOrSpecial(relativePath)) return relativePath;
+  if (!needsRewrite(relativePath)) return relativePath;
+  const compDir = dirname(compSrcPath);
+  if (!compDir || compDir === ".") return relativePath;
+  const resolved = join(compDir, relativePath);
+  const normalized = resolve("/", resolved).slice(1);
+  return normalized;
+}
+
+/**
+ * Rewrite all relative `src` and `href` attributes on elements within a
+ * DOM tree, adjusting paths from the sub-composition's directory context
+ * to the project root.
+ *
+ * @param elements - Iterable of DOM elements to scan (e.g. from querySelectorAll)
+ * @param compSrcPath - The `data-composition-src` value
+ * @param getAttr - Function to read an attribute from an element
+ * @param setAttr - Function to set an attribute on an element
+ */
+export function rewriteAssetPaths<T>(
+  elements: Iterable<T>,
+  compSrcPath: string,
+  getAttr: (el: T, attr: string) => string | null | undefined,
+  setAttr: (el: T, attr: string, value: string) => void,
+): void {
+  const compDir = dirname(compSrcPath);
+  if (!compDir || compDir === ".") return;
+
+  for (const el of elements) {
+    for (const attr of PATH_ATTRS) {
+      const val = (getAttr(el, attr) || "").trim();
+      if (isAbsoluteOrSpecial(val)) continue;
+      if (!needsRewrite(val)) continue;
+      const rewritten = join(compDir, val);
+      const normalized = resolve("/", rewritten).slice(1);
+      if (normalized !== val) {
+        setAttr(el, attr, normalized);
+      }
+    }
+  }
+}

packages/core/src/index.ts

@@ -120,6 +120,7 @@ export type {
   HyperframeLinterOptions,
 } from "./lint/types";
 export { lintHyperframeHtml } from "./lint/hyperframeLinter";
+export { rewriteAssetPaths, rewriteAssetPath } from "./compiler/rewriteSubCompPaths";
 
 // Inline scripts
 export {