midi2taud: another attack-filterenv fix

CLAUDE.md

@@ -196,7 +196,9 @@ The Taud playback engine lives in `tsvm_core/src/net/torvald/tsvm/peripheral/Aud
 
 **Per-patch envelopes go through the Voice's ACTIVE-envelope view, never `inst.*` directly.** Since 2026-06-13 an Ixmp patch can carry its own volume / pan / filter / pitch envelopes (+ fadeout / cutoff / resonance) — see terranmon.txt §Ixmp, variable-length patches. `applyActiveSample` → `resolveActiveEnvelopes(voice, inst, patch)` snapshots the effective envelope source onto `voice.active{Vol,Pan,Pitch,Filter}Env{,Loop,Sustain}`, `voice.has{Pitch,Filter}Env`, and `voice.active{FadeoutStep,DefaultCutoff,DefaultResonance}`. The base instrument exposes **two** pf-envelope slots — bytes 19.. (`pfEnv*`) and bytes 197..250 (`pf2Env*`, the mandatory complement) — routed into the pitch/filter roles by each slot's m-bit (LOOP-word bit 7). `advanceEnvelope` (vol+pan), `advancePitchEnvelope`, `advanceFilterEnvelope`, `applyKeyLift`, the per-tick pitch/filter/fadeout application (foreground AND background), and `triggerNote`'s envelope seeds must ALL read the `voice.active*` view, not `inst.*`. `copyVoice` (NNA ghost) must copy the whole active view so ghosts keep their patch's envelopes. There is no single `envPf*`/`envPfIsFilter` field any more — it was split into explicit `envPitch*`/`envFilter*` pairs. Headless coverage: `devtests/ixmp/PatchEnvTest` (per-patch env applied) + `IxmpFileTest /tmp/m_e1m1.taud`.
 
-**The shared pitch/filter envelope walker (`advancePfRole`) must SKIP zero-duration nodes, not freeze on them.** A node whose `offset` rounds to 0 — sub-4 ms, since `ThreeFiveMinifloat`'s smallest non-zero step is ≈3.9 ms — represents an instant transition; the walk must advance to the next node. The old code `return`ed on `offset == 0.0` without advancing the index, stranding fast-attack envelopes at their first node. The audible damage: SF2 filter mod-envelopes (`midi2taud.py` `_filter_env_block_sf`) routinely have a ~1 ms attack that stores offset 0, so the filter never opened from its base cutoff to its sustain cutoff — Strings/Flute/Guitar (SGM base ~600 Hz, sustain ~6 kHz) and low-base sweep drums played permanently muffled at their floor. The skip loop stops at a sustain/loop boundary (`susEnd`, handled by the dispatch above) or `maxIdx`. This also affects pitch mod-envs and any IT/XM envelope with a zero-tick (vertical-jump) node, all now correct. There is still a one-tick (≈seed) delay before the env opens — inaudible on sustained notes; the seed value is the base node.
+**The shared pitch/filter envelope walker (`advancePfRole`) must SKIP zero-duration nodes, not freeze on them.** A node whose `offset` rounds to 0 — sub-4 ms, since `ThreeFiveMinifloat`'s smallest non-zero step is ≈3.9 ms — represents an instant transition; the walk must advance to the next node. The old code `return`ed on `offset == 0.0` without advancing the index, stranding fast-attack envelopes at their first node. The audible damage: SF2 filter mod-envelopes (`midi2taud.py` `_filter_env_block_sf`) routinely have a ~1 ms attack that stores offset 0, so the filter never opened from its base cutoff to its sustain cutoff — Strings/Flute/Guitar (SGM base ~600 Hz, sustain ~6 kHz) and low-base sweep drums played permanently muffled at their floor. The skip loop stops at a sustain/loop boundary (`susEnd`, handled by the dispatch above) or `maxIdx`. This also affects pitch mod-envs and any IT/XM envelope with a zero-tick (vertical-jump) node, all now correct.
+
+**The note-on / Q-retrigger SEED must also settle past leading zero-duration nodes (`seedPfRole`), not capture node 0.** The trigger code used to seed `envFilterValue`/`envPitchValue` at `activeFilterEnv[0]` and only let `advancePfRole` skip the zero node on the NEXT tick — a one-tick hold at the base node. Inaudible on a sustained note (the old "≈seed delay" caveat), but on a PERCUSSIVE instrument that one tick is the whole attack transient: GeneralUser-GS Slap Bass (PASSPORT.MID) has a 1 ms (offset-0) filter-mod attack opening base→peak then a 0.7 s decay to a mellow sustain — the slap should be BRIGHT then mellow, but the seed played the muddy base cutoff (~507 Hz) for tick 0 then "suddenly opened" to full brightness. `seedPfRole` runs the walker with `tickSec = 0` / `keyOff = false` at note-on (and on Q-retrigger) so the seed lands on the post-attack value (index settled past the dur-0 nodes); an env with a real (non-zero) attack is unchanged. The sample start-offset is a red herring (it was 0) — a quiet sample lead-in would only MASK the muddy tick, which is why it surfaced on slap bass. The vol/pan walker (`advanceEnvelope`) intentionally FREEZES on zero-offset nodes (IT terminator semantics) and is NOT seeded this way.
 
 **SoundFont filter mode uses an RBJ biquad, NOT the IT all-pole filter.** `refreshVoiceFilter` has two topologies. The IT/tracker path (`else` branch) is the all-pole 2-pole resonant LPF from `reference_materials/tracker_filter/` (no feedforward zeros) — must stay byte-faithful for tracker playback, do not touch it. The **`filterSfMode` branch ports FluidSynth's voice filter** (`reference_materials/fluidsynth/`, see its `README.md`): cutoff = absolute cents → Hz via `8.176·2^(cents/1200)` clamped to `[5 Hz, 0.45·fs]`; Q from centibels with FluidSynth's **−3.01 dB offset** (so Q=0 cB ⇒ q_lin = 1/√2 Butterworth, no resonance hump); RBJ cookbook low-pass coefficients with the SF2 `1/√Q` passband gain-norm. `applyVoiceFilter` runs the biquad (Direct Form I: `y = b02·(x+x₂) + b1·x₁ − a1·y₁ − a2·y₂`) when `voice.filterIsBiquad`. The old code reused the all-pole filter for SF mode too; it is overdamped and rolled the passband off ~3 dB @ 8 kHz / ~5 dB @ 12 kHz vs FluidSynth → audible muffling on every filtered GM instrument. Per-voice biquad state (`filterBqB02/B1/A1/A2`, input history `filterX1/X2`) must be reset on trigger/retrigger and copied in `copyVoice` (NNA ghost) alongside the output history. The background-voice filter-env path must branch on `filterSfMode` too, else an SF-mode ghost's cents-domain cutoff gets clamped into the IT 0..254 byte range (≈9 Hz → silence).