docs: extensive PDA payload investigation, split-region semantics resolved (#2)

CHANGELOG.md

@@ -16,6 +16,43 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   see `docs/format/06-known-limitations.md` #9). Resolves
   Sigilweaver/OpenSZRaw#9.
 
+### Documentation
+
+- Further PDA/chromatogram payload investigation
+  (Sigilweaver/OpenSZRaw#2, contributed by @Nabejo): seven same-day
+  sessions of clean-room analysis. Confirmed findings: 2 of the 4
+  varying `PDA 3D Raw Data/CheckSum` fields are exact stream byte sizes
+  (correcting an earlier "flat vs. real flag" reading), and the
+  "split" envelope form's two regions are an exact
+  256-wavelength-channel/remainder split (also explaining why "split"
+  vs. "symmetric" form correlates with wavelength count). Ruled out, in
+  each case run through actual value decoding, a physical-plausibility
+  check, and randomized controls rather than reported on walk-rate
+  alone: a 19-polynomial CRC-16 sweep and several count/derived-size
+  candidates for the remaining `CheckSum` fields; a fixed-width fp16
+  array; a block-floating-point/adaptive-scale hypothesis family; a
+  region-tail marker-bit signal that passed two randomized controls but
+  traced to a compensating-error artifact; two more dramatic-looking
+  signals surfaced by re-running sweeps with corrected per-region
+  target counts; an anti-mode-collapse cost term added to the joint
+  temporal+magnitude decoder; from a deliberately manual (not
+  automated-sweep) byte-reading pass, a "leading byte of a 3-byte
+  token" hypothesis that traced to the same low-value-diversity metric
+  artifact on three independent files; and, cross-referencing the
+  PSI-MS/mzML open spec directly, two MS-Numpress-inspired
+  nibble-granular varint encodings and literal zlib/DEFLATE framing of
+  the payload - the one nonzero hit rate found (a nibble scheme on one
+  file) was disqualified by a shuffled-byte control and cross-file
+  testing, and DEFLATE's small hit rate proved statistically
+  indistinguishable from random-byte and shuffled-byte controls. Along
+  the way, quantified a ~48% false-positive base rate for this
+  document's zero-leftover acceptance test and fixed a real gap in the
+  physical-plausibility check itself (mode-dominated/low-diversity
+  decodes can look deceptively "smooth" under mean relative step
+  alone). The per-point payload grammar itself is still undecoded - see
+  `docs/format/04-lcd-chromatogram-pda.md`'s 2026-07-20 sessions 1-7 for
+  full detail.
+
 ## [0.1.0] - 2026-07-18
 
 ### Added

CONTRIBUTORS.md

@@ -2,4 +2,5 @@
 
 Thank you to everyone who has contributed to OpenSZRaw.
 
-No external contributions yet - this list will grow as people help out.
+- [@Nabejo](https://github.com/Nabejo) - PDA/chromatogram payload
+  investigation (Sigilweaver/OpenSZRaw#2).