feat: Phase 17.3 — Fixed embed queue panel with collapse/expand iframe resize (OQ1 Option A)

Read-only inline queue panel below the release embed's player bar; row-jump reuses PlayRelease. ForRelease mints a taller iframe plus a postMessage resize listener for the collapse toggle; ForTrack unchanged.

DeepDrftPublic/Interop/embed/embed-frame.ts

@@ -0,0 +1,31 @@
+// Embed (iframe) → host resize handshake (Phase 17 wave 17.3, OQ1 Option A).
+//
+// The Fixed-mode player renders an always-shown queue panel below the controls. A collapse/expand
+// toggle lets the embedder's viewer reclaim the panel's vertical space — but collapsing inside the
+// iframe only reclaims space if the *outer* iframe element also shrinks. The iframe cannot resize
+// itself, so it posts its desired pixel height to the host page; the embed snippet (minted by
+// EmbedSnippetBuilder.ForRelease) carries a tiny listener that sets iframe.style.height.
+//
+// Degrades safely: if the host page ignores or strips the snippet's listener (Option B's value), the
+// panel still renders and toggles inside the iframe — only the outer resize is lost. We post nothing
+// when not framed (window === parent), so the docked player is unaffected.
+
+const MESSAGE_TYPE = "deepdrft-embed-resize";
+
+/**
+ * Measure the live rendered height of the player element and post it to the host page so it can size
+ * the iframe to match. No-op when not embedded in a frame, or when the element is unmeasurable.
+ *
+ * targetOrigin is "*" deliberately: the embedder's origin is unknown (any blog can embed us) and the
+ * payload carries no secrets — just a height the host is free to ignore.
+ */
+export function postHeight(element: HTMLElement): void {
+    if (window.parent === window) return; // Not framed — nothing to resize.
+    if (!element) return;
+
+    // ceil + a hairline guard against sub-pixel rounding that would otherwise clip the bottom edge.
+    const height = Math.ceil(element.getBoundingClientRect().height) + 2;
+    if (!Number.isFinite(height) || height <= 0) return;
+
+    window.parent.postMessage({ type: MESSAGE_TYPE, height }, "*");
+}