refactor(capture): drop master scope; realtime taps selected track

Capture is now item + track only (master removed as a scope; still bypassed
as out-of-scope chain). Realtime records the selected track's own output via
per-track post-fader sends into a hidden temp track, fixing the silent file.

src/capture.h

@@ -21,6 +21,12 @@
 
 #include "bank_model.h"
 
+// MediaTrack is forward-declared (like track_guid.h) so this header stays
+// REAPER-free while RealtimeRecordBackend::begin can take the resolved source
+// MediaTrack* to tap. The pointers are opaque here — never dereferenced in a
+// pure/header context; only the REAPER-facing capture_realtime.cpp touches them.
+class MediaTrack;
+
 namespace reasampler {
 
 // Audio bit-depth for the rendered wav. 32-bit float is the M3 default —
@@ -150,8 +156,9 @@ struct RealtimeTickResult {
 };
 
 // The opaque in-flight capture state. Owns the snapshot of everything to restore
-// (temp track, other tracks' I_RECARM, master send/routing, transport, edit cursor,
-// time selection) and the record's own project handle. Defined in
+// (temp track + its receive sends from the source tracks, other tracks' I_RECARM,
+// transport, edit cursor, time selection) and the record's own project handle.
+// Defined in
 // capture_realtime.cpp; the header stays REAPER-free (no MediaTrack*/ReaProject*
 // leaks here) by holding it behind a forward-declared type + unique_ptr.
 //
@@ -182,21 +189,25 @@ using RealtimeCaptureHandle =
 // harder here than offline because the record spans ticks: the snapshot + restore
 // live on RealtimeCaptureState, not a function-scope RAII destructor.
 //
-// SCOPE (this increment): MASTER scope only — records the master-mix output, which
-// does not need the FxBypassGuard scope isolation (the whole chain is in scope).
-// Track/Item scopes are a genuine routing fork surfaced to Daniel, NOT silently
-// built (see capture_realtime.cpp §FORK). A Track/Item request is refused.
+// SCOPE (this increment): TRACK scope only — records the selected track's OWN
+// output (item + that track's own FX + its own fader/pan, PRE-parent), matching
+// offline's track scope. This needs NO FxBypassGuard: a send tapping a track's
+// output is naturally PRE-parent (the parent has not summed it yet), so the tap is
+// chain-independent by construction. Item realtime is deferred (UnsupportedMode).
 class RealtimeRecordBackend {
 public:
-    // Starts a realtime record: validates the request (master scope, non-empty
-    // range, active + saved project, transport idle), snapshots all state to
-    // restore, creates the hidden temp track, routes the master send, arms, and
-    // CSurf_OnRecord — then returns IMMEDIATELY (no wait, no UI block). On success
-    // the returned unique_ptr owns the in-flight state; drive it with tick(). On a
-    // validation/setup failure returns nullptr and fills `outFailure` with the
-    // CaptureStatus + message (nothing was left mutated — begin() restores on its
-    // own failure paths).
+    // Starts a realtime record: validates the request (track scope, non-empty range,
+    // at least one source track, active + saved project, transport idle), snapshots
+    // all state to restore, creates the hidden temp track, routes a send FROM each
+    // source track INTO the temp track, arms, and CSurf_OnRecord — then returns
+    // IMMEDIATELY (no wait, no UI block). `sourceTracks` are the selected tracks to
+    // tap (resolved by the action layer — the CaptureRequest itself stays REAPER-free,
+    // carrying only the provenance GUIDs). On success the returned unique_ptr owns the
+    // in-flight state; drive it with tick(). On a validation/setup failure returns
+    // nullptr and fills `outFailure` with the CaptureStatus + message (nothing was
+    // left mutated — begin() restores on its own failure paths).
     RealtimeCaptureHandle begin(const CaptureRequest& request,
+                                const std::vector<MediaTrack*>& sourceTracks,
                                 CaptureResult& outFailure);
 
     // Advances the in-flight record one tick. Reads the transport (bound to the