docs: NOTES_LEVEL update — LUT level-domain discovery, sweep tooling hazard

handoff/NOTES_LEVEL.md

@@ -1182,3 +1182,42 @@ cut. => Naive additive/sens formulas CANNOT model the negative-sens band interac
 lives in the structural combine/acc 0x5407c8 (per-bin weights kRTAtt/kRTRel + negative
 sens entering the level path with opposite sign), which requires the real exp2-domain
 chain, not the bridge. comb stays OPEN (structural combine/acc, P4/P5).
+
+## ============ UPDATE 2026-08-21: LUT moved to LEVEL domain (structural chain) ============
+### Discovery: mask-domain LUT clamps on loud input
+- Mask-domain LUT (committed 9b7e9d3) computes t=(mask_dB-A)/(B-A); for loud input
+  (tone1k 0dBFS) post-exp2 mask_dB < A=-13.78 => t clamps to 0 => val=0 => FULL cut.
+  Measured t1k err -24.04 dB. Root cause: exp2(-level) output dB range is unbounded
+  below, LUT domain [A,B] assumes bounded input.
+- FIX: apply FUN_180563a60 power-law to LEVEL (am/res*scale) BEFORE IIR/exp2:
+  lvl = pow(t, gamma)*MULT, t=(20*log10(lvl)-A)/(B-A) clamp [0,1].
+  Level dB stays inside [A,B] for both quiet (t1kq: +6.2dB) and loud (t1k: +24dB).
+  Result: t1k err -24.04 -> +2.45 dB.
+
+### TOOLING HAZARD: stale-binary sweeps
+- cmake --build skips recompile when source mtime unchanged within same second;
+  param sweeps silently re-run the SAME binary (9-point sweep -> 1 unique result).
+  Earlier "gamma doesn't matter" observation was this artifact, NOT DSP behavior.
+- Protocol now: os.utime(src) before build + assert binary mtime fresh (sweep_fresh.py).
+
+### Fresh 2D sweep (g x mult, verified builds), 3-case smoke (render48k 48k/4096):
+  g=0.344 m=3.8 optimum: t1kq=-0.788 t1k=+2.453 al12=+0.037 mean|err|=1.093
+  (committed version was mean 8.2). gamma>=0.9 ALL worse (+2..+5): spatial IIR
+  after LUT redistributes peaks; naive ratio math (need g=1.17 from raw t ratio
+  1.744->2.0) does NOT survive the IIR chain. Empirics rule.
+- MULT=3.8 is a PLACEHOLDER (maps t^gamma back to level domain; no decomp basis yet).
+  A/B/gamma are decomp-extracted (BandConfig ctx+0x188 refs, NOTES:967).
+
+### Scope clarification
+- scripts/corpus.py exercises the BRIDGE path only (44.1k non-internal grid):
+  full corpus 1.594 mean / comb 10.15 — UNCHANGED by structural work (verified
+  stash-compare). comb regression is pre-existing bridge state, separate issue.
+- Structural chain validated via render48k smoke (3 cases) + module checks
+  (fn529fe0/exp2/twin/tables/leveltrack/levelpath/fftconv all PASS).
+- render48k tail artifact: last BLK=65536 block zero-pads => am decays in final
+  frames (~last 0.06s of y48). Cosmetic; metric window barely affected.
+
+### Commit: 535b150 (LUT level-domain restructure).
+NEXT: step 3 wire combine_acc track into mask output (currently computed,
+discarded); step 4 FFT-conv 0x535a70 mask smoothing; then decode BandConfig
+ctx+0x188 writers to replace MULT placeholder with decomp value.