feat: add tempo-synced stereo delay and coupled room reverb

docs/BUILTIN_DEVICES.md

@@ -121,3 +121,34 @@ license/provenance in SAMPLE_LICENSE.md. Both are installed with the examples.
 Tests cover source-rate conversion, one-shot endpoint, looping, release,
 partitioning, preset references, missing assets, GUI relinking/undo/reload and
 preparation-allocation/callback guards.
+
+## Phase 43 stereo effects
+
+**File → Add stereo delay/reverb** and **Edit → Insert stereo delay/reverb on
+wire** add new devices (indices 5/6). Original Delay/Reverb (indices 2/3) retain
+their original algorithms, values and saved identities. Existing control and
+automation editors, undo and persistence support both generations.
+
+Stereo Delay adds integer **Sync**: 0 milliseconds, 1 quarter, 2 eighth,
+3 dotted eighth, 4 eighth triplet. Nonzero sync follows project tempo; changing
+tempo uses ordinary stopped project preparation. Time changes crossfade two
+fractional read heads for 5 ms. A crossfade finishes before adopting the latest
+automated target, bounding transition work and avoiding read-head jumps.
+Feedback cross-couples channels (80% same, 20% opposite), with a 0.95 bound.
+Delay storage covers four seconds, sufficient for all divisions at 20..300 BPM.
+
+Stereo Reverb adds **Room** (0..1, scales comb spacing) and **Diffusion** (0..1,
+cross-couples damped feedback). The right comb timings differ from the left.
+Decay/damping retain 0..0.95 bounds. This is a simple algorithmic reverb, without
+physical room simulation or a measured RT60. Room automation can alter coloration.
+
+The graph evaluates both channels' inputs before calling a coupled internal
+processor. Each stereo device has one prepared state, with no callback allocation.
+Mono processing excites both sides and folds the wet output to mono. External
+plugins and the original effects retain independent mono instances.
+
+Tests compare whole/split blocks, tempo-synced impulse placement, left-to-right
+energy, bounded extreme feedback/diffusion and graph/direct stereo output
+(including the engine's subnormal-to-zero policy). GUI tests save/reopen both
+identities; callback guards and ASan/UBSan cover processing. `make builtin-preview`
+also writes files 5/6 for repeatable listening; human listening remains pending.

docs/ROADMAP.md

@@ -1217,15 +1217,19 @@ See BUILTIN_DEVICES.md for memory/format and transposition limits.
 Address the documented sound-design and transition limits of the basic effects.
 
 ### Tasks and acceptance criteria
-- [ ] Smooth or crossfade delay-time changes and define automation behavior without abrupt read-head jumps.
-- [ ] Add tempo-synced delay divisions with deterministic tempo/seek behavior.
-- [ ] Add true stereo processing and reverb diffusion/room controls rather than independent mono instances alone.
-- [ ] Preserve old projects' effect identities, parameters and sound, or provide an explicit compatibility mode.
-- [ ] Test stability at parameter extremes, stereo response, automation partitions and transition artifacts; document listening results.
+- [x] Smooth or crossfade delay-time changes and define automation behavior without abrupt read-head jumps.
+- [x] Add tempo-synced delay divisions with deterministic tempo/seek behavior.
+- [x] Add true stereo processing and reverb diffusion/room controls rather than independent mono instances alone.
+- [x] Preserve old projects' effect identities, parameters and sound, or provide an explicit compatibility mode.
+- [x] Test stability at parameter extremes, stereo response, automation partitions and transition artifacts; document listening results.
 
 ### Implementation status
-Planned. Delay time can click; reverb uses four damped mono combs without a room
-model or stereo diffusion. Current decay is a feedback coefficient, not seconds.
+Implemented as new Stereo Delay/Reverb identities, preserving original devices.
+Delay uses fractional read-head crossfades and tempo divisions; reverb adds room
+spacing and coupled stereo diffusion. Graph evaluation now processes coupled
+channels together. Core, graph/direct DSP, partition, tempo, GUI persistence,
+callback-guard and sanitizer tests pass. Listening previews/procedure are
+documented; human listening is unverified. See BUILTIN_DEVICES.md.
 
 ---