codec-re: handoff polish — readmes, skeleton, remove decode-re/ duplicate

Three things to make pickup smoother:

1. analysis/README.md (NEW): catalogues the ~25 scratch scripts.
   Categorizes them as "still useful" / "superseded — keep for
   archaeology" / "pure exploration".  Tells a fresh engineer which
   files to read first and which to ignore.

2. scratch/next_experiment_skeleton.py (NEW): stub + spec for the
   segment-channel scoring analyzer.  Includes the fixture loader,
   block walker, and decode-segment-as-channel helper — just enough
   scaffolding that the next pass starts from "fill in
   score_segment_against_all_channels()" rather than from scratch.
   Already runs and confirms 13 segments per 3-sec event with sample
   starts going to 6590 (way past the 3328 actual samples) — strong
   evidence that not all segments carry Tran.

3. Removed decode-re/ duplicate.  It was a mirror of tests/fixtures/.
   Analysis scripts that hardcoded decode-re/ paths updated to point
   at tests/fixtures/.  CLAUDE.md note updated: future event uploads
   go directly into a dated subdirectory under tests/fixtures/.

All 40 tests still pass.  Skeleton runs.

CLAUDE.md

@@ -134,11 +134,16 @@ in `minimateplus/waveform_codec.py` returns `None` as a placeholder.
 ### Test fixtures
 
 `tests/fixtures/decode-re-5-8-26/` and `tests/fixtures/5-11-26/` —
-seven BW binary + ASCII pairs captured from a live BE11529.  The
+nine BW binary + ASCII pairs captured from a live BE11529.  The
 5-11-26 high-amplitude bundle (PPV 6–7 in/s) is what cracked the Tran
 codec; the V70 (mic-heavy) + JQ0 (Vert-heavy) pair cracked the `00 NN`
 RLE rule.
 
+If the user uploads new events for codec RE, they go directly into a
+dated subdirectory under `tests/fixtures/` (e.g. `tests/fixtures/5-18-26/`).
+There used to be a separate `decode-re/` upload mirror but it was
+removed once the fixtures directory became the canonical location.
+
 ---
 
 ## Protocol fundamentals