fix(slideshow): finish remaining split-PR review findings (#1594)

* fix(slideshow): address split-PR review findings on #1585

Genuinely-open findings from the #1580/#1590/#1591/#1592 reviews (the rest were
already fixed on this branch: CSP handlers, manifest version, UUID ids, float
keys, presenter 1s-timer):

core (#1580):
- isManifest rejects a non-object/array manifest (e.g. [42,null]) explicitly
- resolveSlideshow flags duplicate slideSequence ids instead of silent overwrite

player (#1590):
- present() window.open uses noopener,noreferrer (audience syncs via channel)
- BroadcastChannel name is per-deck (keyed on pathname) to avoid same-origin
  cross-talk between decks
- add observedAttributes + attributeChangedCallback so runtime sound/mode toggles
  re-render

studio (#1591/#1592):
- persistSlideshowManifest no-op gate (skip write when HTML is unchanged)
- surface persist failures (console.error) instead of silent .catch(()=>{})
- confirm before deleting a branch sequence (data-loss + dangling hotspots)

+ tests for the collision + non-object-manifest rejection. 20 core / 106 player /
53 studio pass; tsc/lint/fmt/fallow clean; deck still renders.

* fix(slideshow): finish remaining split-PR review findings

The larger items from the #1580/#1590/#1591/#1592 reviews (the rest landed in
#1585):

core (#1580):
- dedup isSceneLikeCompositionId — shared slideshow/sceneId.ts, used by both the
  lint rule and the runtime scene-window computation (no more mirror-and-drift)

player (#1590 / #1592):
- onKey: when multiple decks share a page, drop the unfocused-convenience so a
  key drives only the focused deck
- slow-iframe recovery: if the scene timeline posts after the wait times out
  (empty scenes), re-init once so sceneId slides resolve instead of being dropped

studio (#1591):
- persistSlideshowManifest validates the built island round-trips before writing
- reorderBranchSlide helper + BranchTree up/down controls (parallel to main-line
  reorder), with a branch-position indicator

+ tests for reorderBranchSlide. core 228 / player 106 / studio (panel) 46 pass;
tsc/lint/fmt/fallow clean.

* fix(player,cli): use fileURLToPath for path resolution (Windows CI)

new URL(...).pathname yields a leading-slash drive path ("/D:/...") on Windows,
which broke:
- packages/player/vitest.config.ts — the @hyperframes/core/slideshow alias
  resolved to a nonexistent path, failing the player slideshow tests on the
  Windows render-verification CI (passed on macOS/Linux where pathname is clean)
- packages/cli/src/utils/compositionServer.ts helperDir — same bug in the
  play/present bundle-path resolution

fileURLToPath converts file:// URLs to correct OS paths on all platforms.
Player slideshow tests pass; present serves + resolves bundles.

* fix(producer): fileURLToPath for the renders dir (Windows)

DEFAULT_RENDERS_DIR used new URL(import.meta.url).pathname, which is "/D:/..."
on Windows and resolves to a bogus path — affects the Windows render pipeline.
Last of the .pathname -> fileURLToPath fixes (repo-wide src sweep now clean).

packages/studio/src/utils/setSlideshowManifest.ts

@@ -20,6 +20,7 @@ import type { SlideshowManifest } from "@hyperframes/core/slideshow";
 import {
   SLIDESHOW_ISLAND_TYPE,
   SLIDESHOW_MANIFEST_VERSION,
+  parseSlideshowManifest,
   slideshowIslandRegex,
 } from "@hyperframes/core/slideshow";
 import type { Composition } from "@hyperframes/sdk";
@@ -64,6 +65,18 @@ export async function persistSlideshowManifest(args: PersistSlideshowArgs): Prom
   const { manifest, sdkSession, originalContent, targetPath, deps, label, coalesceKey } = args;
 
   const islandHtml = buildSlideshowIslandHtml(manifest);
+
+  // Write-time validation: confirm the island we just built round-trips to a
+  // valid manifest before touching disk, so a malformed edit can't corrupt the
+  // composition. parseSlideshowManifest throws on a structurally-invalid island.
+  try {
+    if (!parseSlideshowManifest(islandHtml)) {
+      throw new Error("built island did not parse back to a manifest");
+    }
+  } catch (err) {
+    throw new Error(`refusing to persist invalid slideshow manifest: ${(err as Error).message}`);
+  }
+
   const current = sdkSession.serialize();
 
   // Strip ALL existing islands (handles the case where two stale islands
@@ -78,6 +91,10 @@ export async function persistSlideshowManifest(args: PersistSlideshowArgs): Prom
     after = stripped + "\n" + islandHtml;
   }
 
+  // No-op gate: if the rewritten HTML is byte-identical to the current serialized
+  // HTML, skip the write — avoids a spurious disk write and a no-op undo entry.
+  if (after === current) return;
+
   await persistSdkSerialize(after, targetPath, originalContent, deps, {
     label: label ?? "Edit slideshow",
     ...(coalesceKey ? { coalesceKey } : {}),