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   Categorizes them as "still useful" / "superseded — keep for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.

analysis/verify_tran.py

@@ -17,7 +17,7 @@ def i8(b):
 
 def main():
     for stem in ("M529LL1A.SP0", "M529LL1A.SS0", "M529LL1A.SV0"):
-        path = f"decode-re/5-11-26/{stem}"
+        path = f"tests/fixtures/5-11-26/{stem}"
         with open(path, "rb") as f:
             raw = f.read()
         body = raw[43:-26]