Ψ-W1-T2: disjoint per-mode solo surfaces and a playback-gated mode switch

Solo is cached, cleared and replayed per mode on a real switch only; the switch is refused visibly while the transport runs. The footer segment now routes through the activate actions, so a panel switch finally persists.

docs/product/design-view.md

@@ -157,12 +157,19 @@ load hitch and any un-persisted internal state is lost. This is an accepted cost
 of the CPU reclaim, not a bug. It must be documented at the toggle affordance so
 the user isn't surprised.
 
-**Never touched:** `B_MUTE` and `I_SOLO`. The tool owns visibility, `B_MAINSEND`,
-`I_FXEN`, and FX-offline — nothing else — across every managed leaf, tagged or
-untagged. The user's mute/solo survives every toggle, untouched. This is the exact
-analog of the
-capture pillar's non-destructive invariant: **the tool never destroys the user's
-real state to do its job.**
+**Never touched:** `B_MUTE`. The tool owns visibility, `B_MAINSEND`, `I_FXEN`,
+FX-offline, and `I_SOLO` — nothing else — across every managed leaf, tagged or
+untagged.
+
+**Solo is owned but never lost.** Solo is a per-mode surface: switching modes banks
+the outgoing mode's solo state, clears it, and replays the incoming mode's on
+return, verbatim. Two modes therefore never share a solo — you can solo the drum
+bus in Arrange and the sound-design chain in Design without either leaking into the
+other — and neither is destroyed. That is the same exact analog of the capture
+pillar's non-destructive invariant the flags above satisfy: **the tool never
+destroys the user's real state to do its job.** It is snapshot-and-restore, one
+level out from a single toggle to the pair of stances. Reapplying the current mode
+(tagging, project load) is not a switch and does not touch solo at all.
 
 ---
 
@@ -316,8 +323,10 @@ Mirrors the capture pillar's split exactly.
 - Snapshots prior flag values before parking (reads the same flags it will drive).
 - Resolves track GUIDs via `GetTrackGUID` / `guidToString` / `stringToGuid` for the
   index; never uses track index (unstable across reorders).
-- Never touches the master track's visibility flags; never touches `B_MUTE` /
-  `I_SOLO` on anything.
+- On a real switch only, banks/clears/replays `I_SOLO` per the per-mode solo surface
+  above.
+- Never touches the master track's visibility flags; never touches `B_MUTE` on
+  anything.
 
 **`persist` slice:**
 - Serialize/deserialize the view section (modes + membership + show-both + snapshots
@@ -585,11 +594,12 @@ The settled distinction:
   touch them**: a mode toggle never shows, hides, silences, re-lanes, or re-plays a
   manual lane. Its `C_LANEPLAYS` state is the user's, left exactly as they set it.
 
-This is the fixed-lane analog of the two invariants already load-bearing in D1 —
-*never touch `B_MUTE`/`I_SOLO`* and *never touch the master* — extended to a third
-surface: **never drive a lane the tool did not mint.** It is the same non-destructive
-promise (the tool owns only what it created) reaching one level deeper, into the lane
-dimension.
+This is the fixed-lane analog of the invariants already load-bearing in D1 —
+*never touch `B_MUTE`*, *never touch the master*, and *never lose the user's solo*
+(see "Never touched" above) — extended to a further surface: **never drive a lane
+the tool did not mint.** It is the same non-destructive promise (the tool owns only
+what it created, and restores what it parks) reaching one level deeper, into the
+lane dimension.
 
 ### Lane-ownership index (the new data)