feat(panes): core detachable pane system + pop-out chip

The option-heavy player UIs (mixer, camera director, viz, audio routing)
all live in the rail popovers, which are exclusive: openPopFor() closes
the last one before opening the next. You cannot watch the mixer while
riding the camera, and both vanish the moment you look at the highway.

Add `window.feedBack.panes` — a core registry for live UI that is
authored once as `mount(root, ctx)` and hosted anywhere. Panes are
non-exclusive, and they survive song switches structurally: the dock is a
body child outside every .screen, so the per-song teardown never sees it.

The adoption cost for a plugin is two calls:

    feedBack.panes.register({ id, title, icon, mount, unmount });
    feedBack.panes.attachChip(myExistingDialogEl, id);

attachChip injects THE standard pop-out chip. Clicking it opens the pane
and hides the plugin's dialog, leaving a stub to bring it back. Core owns
the hide/restore, so every plugin's pop-out looks and behaves the same —
which is the point. It hides via a dedicated .fb-pane-detached class, not
.hidden/[hidden], because the dialogs we attach to already toggle those.

Everything a pane may touch arrives through `ctx` — never a global. That
is what will let the same mount() run inside a pop-out window, a separate
JS realm with no window.feedBack, no window.highway and no audio graph:

  ctx.call(domain, cmd, payload)  -> the capability bus
  ctx.on(event, fn)              -> the feedBack bus (allowlisted)
  ctx.subscribe(stream, fn)      -> playhead / meters
  ctx.state.get/set              -> persisted, main realm is the only writer
  ctx.playhead(), ctx.song(), ctx.toast(), ctx.close()

ctx tracks every subscription it hands out and drops them on unmount, so
a pane cannot leak listeners across a dock/undock cycle.

Streams exist because an AnalyserNode cannot cross a window boundary:
levels are reduced to numbers in the realm that owns the audio graph.
One shared rAF loop, refcounted against live subscriptions, dirty-checked
before fan-out, and stopped dead when the last pane closes.

Hosts register themselves with the manager rather than being imported by
it — the dock lands at priority 0 (the floor, always available), so the
OS pane window can drop in later without this code changing.

Ships two built-in panes: Now Playing (the reference pane — reads the bus,
a stream, and levels, and touches no globals) and Mixer (the same faders
as the rail, via ctx.call('audio-mix', ...), with the chip attached to the
real #mixer-control). Plus a "Panes" rail popover to open panes that have
no dialog of their own; the system tray will mirror that list.

Note the dock sits at z-index 110, not on the docs/plugin-v3-ui.md ladder
(transport 20, rail 30, popovers 40) — those live INSIDE #player's
stacking context, and #player is itself fixed at z-index 100. A dock below
100 is invisible on the one screen panes exist for. Body-level ladder:
#player 100 < dock 110 < toasts 120 < modals 200.

Pop-out windows, the system tray, manifest-declared panes and mirrorGlobal
(the window.__h3dCamCtl proxy the camera director needs) follow.

Signed-off-by: topkoa <topkoa@gmail.com>

static/panes/pane-launcher.js

@@ -0,0 +1,61 @@
+/*
+ * fee[dB]ack — pane launcher (the "Panes" rail popover).
+ *
+ * A chip only works for a pane that already has a dialog to hide. Panes with no
+ * dialog — a readout, a plugin's optional extra — need somewhere to be opened
+ * from, so every registered pane gets one: a checkbox list in the rail.
+ *
+ * The rail popover is the right home for this precisely because it IS exclusive
+ * and transient. It's a menu, not a workspace; the panes it opens are the
+ * workspace, and they persist.
+ *
+ * Populated from the registry, so a plugin that calls panes.register() appears
+ * here with no further work. (The system tray will mirror this list.)
+ */
+(function () {
+    'use strict';
+
+    const panes = window.feedBack && window.feedBack.panes;
+    const bus = window.feedBack;
+    if (!panes || !bus || typeof bus.on !== 'function') return;
+
+    let listEl = null;
+
+    function render() {
+        if (!listEl || !listEl.isConnected) listEl = document.getElementById('v3-rail-panes-list');
+        if (!listEl) return;
+        const all = panes.list();
+        listEl.replaceChildren();
+
+        if (!all.length) {
+            const empty = document.createElement('div');
+            empty.className = 'v3-pop-empty';
+            empty.textContent = 'No panes available.';
+            listEl.appendChild(empty);
+            return;
+        }
+
+        all.forEach((p) => {
+            const b = document.createElement('button');
+            b.type = 'button';
+            b.className = 'v3-pop-btn';
+            b.setAttribute('aria-pressed', p.open ? 'true' : 'false');
+            b.textContent = (p.open ? '● ' : '○ ') + p.icon + ' ' + p.title;
+            b.addEventListener('click', (e) => {
+                e.stopPropagation();
+                if (panes.isOpen(p.id)) panes.close(p.id); else panes.detach(p.id);
+            });
+            listEl.appendChild(b);
+        });
+    }
+
+    // The registry changes when plugins load and when panes open/close. Render is
+    // cheap and rare (never on a playback path), so just re-run it.
+    bus.on('panes:registered', render);
+    bus.on('panes:unregistered', render);
+    bus.on('panes:opened', render);
+    bus.on('panes:closed', render);
+
+    if (document.readyState !== 'complete') document.addEventListener('DOMContentLoaded', render);
+    else render();
+})();