feat: render bounded effect tails and reset internal history on transport changes

docs/ROADMAP.md

@@ -1239,15 +1239,19 @@ documented; human listening is unverified. See BUILTIN_DEVICES.md.
 Make discontinuities and effect tails explicit in playback and export.
 
 ### Tasks and acceptance criteria
-- [ ] Define retained/reset effect history for seek, stop, loop wrap and project/plan replacement.
-- [ ] Implement the chosen history policy without callback allocation or unsafe state sharing.
-- [ ] Add bounded automatic or user-configurable tail rendering after the arrangement/export endpoint.
-- [ ] Expose tail limits and cancellation, including silent input and non-decaying/high-feedback cases.
-- [ ] Test seek/loop reproducibility and complete delay/reverb tails across processing block sizes.
+- [x] Define retained/reset effect history for seek, stop, loop wrap and project/plan replacement.
+- [x] Implement the chosen history policy without callback allocation or unsafe state sharing.
+- [x] Add bounded automatic or user-configurable tail rendering after the arrangement/export endpoint.
+- [x] Expose tail limits and cancellation, including silent input and non-decaying/high-feedback cases.
+- [x] Test seek/loop reproducibility and complete delay/reverb tails across processing block sizes.
 
 ### Implementation status
-Planned. Seeks currently retain effect history; tails need continued rendering
-and do not automatically extend playback/export.
+Implemented: constant-cost internal history invalidation on discontinuity/stop,
+plus incremental stereo WAV export with 0..60-second tails, progress/cancel and
+no-clobber publication. Preferences optionally stop playback after the tail
+(with GUI-timer/fade precision); loops/recording retain their existing behavior.
+Core, reliability, export/direct partition, GUI and sanitizer checks pass.
+External-plugin history remains retained. See BUILTIN_DEVICES.md for limits.
 
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