Docs: reconcile to two-scope capture model (master removed)

Update the CLAUDE.md FX-scope invariant to item+track only; note master
removal in COMPLETED.md; drop master references from the tail spec and PLAN
Milestone T; realtime now taps the selected track.

COMPLETED.md

@@ -281,5 +281,6 @@ auto-inserts into the arrange.
 
 **Superseded / reworked (post-landing):**
 - The four wet source-mode actions (master/tracks/items/razor) were replaced by **three FX-scope actions** — `capture item`, `capture track`, `capture master` — with range (razor-else-time-selection) inferred orthogonally. This fixed the defect where item captures were rendered through the parent FX chain.
-- **FX-scope semantics:** item = item/take FX only; track = item FX + the selected track's own track FX; master = full chain. For item/track, the out-of-scope chain (ancestors + master, plus the item's own track for item scope) is neutralized during the render.
+- **FX-scope semantics (initial rework):** item = item/take FX only; track = item FX + the selected track's own track FX; master = full chain. For item/track, the out-of-scope chain (ancestors + master, plus the item's own track for item scope) is neutralized during the render.
+- **Master scope subsequently removed:** capture is now **two scopes — item and track only**. `CAPTURE_MASTER` and `CAPTURE_MASTER_REALTIME` are retired (to capture the master, render a track instead). The master track is still neutralized as out-of-scope chain for both item and track captures; it is a bypass target, not a capture scope. The realtime backend taps the selected track (track scope only; item realtime deferred).
 - **`FxBypassGuard` (RAII):** snapshot → neutralize (FX bypassed via `I_FXEN`; gain zeroed via `D_VOL`; pan/width/pan-law/mode set to unity via `D_PAN`/`D_WIDTH`/`D_PANLAW`/`I_PANMODE`) → render → restore. Non-destructive. This guard is the reusable mechanism M8 (realtime backend) and M10 (null-test / true dry) build on.