Edit region: Loop-in-3D round-trip between player and Song Editor (#575)

* Add "Edit region" + Loop-in-3D handoff between player and Song Editor

Wires the player half of the Editor ⇄ 3D Highway region round-trip
(editor half is in feedback-plugin-editor).

Highway → Editor:
- New "✎ Edit region" button in the loop controls (v2 and v3) opens the
  Song Editor scrolled to the active A–B loop — or, when none is set, the
  section under the playhead (or a short window around it).
- A "↩ Editor" button appears after a Loop-in-3D handoff to return to the
  exact edit position you came from.
- Both are hidden unless the editor plugin is loaded (typeof
  window.editSong) and gated by _updateEditRegionBtn.

Editor → Highway:
- A one-shot song:ready listener consumes window._pendingHighwayLoop set
  by the editor's "Loop in 3D" button — after playSong()'s own clearLoop()
  has run — arming setLoop(a,b) over the region and auto-starting playback.
  Filename-guarded so a cancelled handoff can't arm a stale loop on an
  unrelated song.

Reuses the existing A/B loop API; no new looping engine. Buttons added to
both static/index.html (v2) and static/v3/index.html (separate file —
v2 markup doesn't carry over), using already-scanned Tailwind classes.

New globals editRegionInEditor / returnToEditorFromHighway; helpers
_resolveEditRegion / _updateEditRegionBtn.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: topkoa <topkoa@gmail.com>

* fix(loop-in-3d): use canonical window.feedBack namespace (post-#537)

The new song:ready loop-applier landed on the legacy window.slopsmith
alias because the branch predated the slopsmith->feedBack rename (#537).
Normalize it to window.feedBack like the rest of core; the alias would
have worked but leaves the lone slopsmith reference in the file.

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

---------

Signed-off-by: topkoa <topkoa@gmail.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
Co-authored-by: byrongamatos <xasiklas@gmail.com>

static/v3/index.html

@@ -806,6 +806,14 @@
                     </select>
                     <button onclick="deleteSelectedLoop()" id="btn-loop-delete" class="v3-pop-btn hidden" title="Delete selected loop">✕</button>
                 </div>
+                <!-- Editor ⇄ 3D Highway round-trip (mirrors the v2 loop cluster). -->
+                <div class="v3-pop-row">
+                    <span class="v3-pop-label">Editor</span>
+                    <span class="flex items-center gap-1 flex-wrap">
+                        <button onclick="editRegionInEditor()" id="btn-edit-region" class="v3-pop-btn hidden" title="Edit this region in the Song Editor">✎ Edit region</button>
+                        <button onclick="returnToEditorFromHighway()" id="btn-return-editor" class="v3-pop-btn hidden" title="Return to the editor where you left off">↩ Editor</button>
+                    </span>
+                </div>
                 <button onclick="showScreen('home')" class="v3-pop-close" title="Close player">✕ Close player</button>
             </div>
         </div>