Rework capture into three FX-scope actions with inferred range

Replace the four capture modes with item/track/master scope actions. Each infers
its range (razor-else-time) and enforces FX scope via non-destructive
FX-bypass-around-render (RAII I_FXEN snapshot/restore over ancestors + master).
Corrects the defect of items captured through parent FX.

src/capture.cpp

@@ -1,11 +1,14 @@
-// capture.cpp — REAPER-facing offline-render backend (M3 spike).
+// capture.cpp — REAPER-facing offline-render backend (OfflineRenderBackend).
 //
 // Compiled into the reaper_reasampler MODULE. Includes
 // reaper_plugin_functions.h WITHOUT REAPERAPI_IMPLEMENT — main.cpp is the one TU
 // that defines the API pointers; here they are extern (CLAUDE.md §contract).
 //
-// Scope (M7): the full offline source family — master mix / time selection,
-// selected tracks, selected items, razor area — all wet-only with optional tail.
+// Renders a CaptureRequest's source over its requested range. The full three-scope
+// capture family (item / track / master, each over a razor-else-time range) is
+// driven here — all wet-only with optional tail. FX scope is enforced by the
+// caller (via FX-bypass-around-render / FxBypassGuard) before invoking capture;
+// this backend is source-agnostic and does not itself read the DAW selection.
 // Drives the RENDER_* project settings via GetSetProjectInfo / _String
 // (the source-selection bits come from render_settings.cpp, the pure mapping),
 // snapshots and restores every setting it changes (non-destructive), triggers a