feat(mixer): control insert wet mix and chain output gain (phase 88)

docs/ROADMAP.md

@@ -2000,12 +2000,12 @@ Implemented: atomic chain preset save/load reuses the versioned project containe
 Expose per-slot wet/dry and chain output gain with finite bounded values and render tests for endpoints and ordering.
 
 ### Tasks and acceptance criteria
-- [ ] Implement the described workflow with validated errors and safe ownership.
-- [ ] Pass relevant core and/or GUI tests.
-- [ ] Document behavior and limitations; commit this phase separately.
+- [x] Implement the described workflow with validated errors and safe ownership.
+- [x] Pass relevant core and/or GUI tests.
+- [x] Document behavior and limitations; commit this phase separately.
 
 ### Implementation status
-Planned.
+Implemented: selected insert wet amount (0–1) and chain output gain (0–4) controls; parallel dry/wet gain branches and pre-fader output scaling. Track-chain tempo follows project tempo during assignment and changes. Exact dry/wet endpoint and fractional impulse tests, full core/reliability suite and GUI mix/invalid-value/tempo/preset tests passed. Mono mix controls use linear amplitude blending.
 
 ---
 

docs/TRACK_INSERTS.md

@@ -58,3 +58,10 @@ applying it verifies all enabled plugins before replacing the current chain.
 Preset files reference absolute installed binary paths, so another machine must
 provide compatible binaries at those paths. No plugin binaries or opaque state
 chunks are embedded.
+
+**Insert wet** blends the selected effect with its input (0 dry, 1 fully wet).
+**Chain gain** scales the complete chain before the track fader (0–4 linear).
+Edits are prepared and undoable, with finite range validation. Internal sync
+parameters follow project tempo. This is mono amplitude blending; stereo track
+insert layouts, latency compensation and gapless live insert edits are future
+work. Editing stops transport before preparing the replacement plan.