chore: retire the last dead paths and names from the consolidation

Nothing here changes what a tool computes; it changes where tools look.

- tools/re-capture: 33 censuses globbed /work/sylph_extract, a path that has
  existed nowhere since /work became a clone, so they matched nothing and
  printed empty results. They now resolve the disc through a new disc.py
  from $SYLPHEED_DISC and exit loudly without it (the #44 fix, generalised).
  Nine scripts that imported siblings from the retired Reborn checkout or an
  old session scratchpad now import from their own directory. unitgroup.py
  only needs the variable when --pak is not given.
- sylpheed-xex: the loader only ever uses the XEX2 retail key. The dead
  devkit key and a doc comment claiming a devkit fallback that does not
  exist are gone; Project Sylpheed is a retail XEX2, so no XEX1 key either.
- sylpheed-viewer: real_font_rasterizes looked for /tmp/sylph_extract and so
  always skipped. It reads $SYLPHEED_DISC now, and passes against the disc.
- Comments and docs that named xenia-rs, the Reborn repository or /work/*.pe
  as places to look now name sylpheed.db, Canary's ppc_context.h and the
  flat .pe; docs/re/README.md no longer says the native Canary build does not
  run.

Historical records keep their original paths: findings that were measured
against /work/xenia-rs/sylpheed.db still say so.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tools/re-capture/whatchanges.py

@@ -8,10 +8,11 @@ capture: constant, smoothly varying (a continuous quantity), or jumpy.
 Usage: whatchanges.py <probe.bin> [name-substring] [max-report]
 """
 import math
+import os
 import struct
 import sys
 
-sys.path.insert(0, "/home/fabi/RE - Project Sylpheed/sylpheed-reborn/tools/re-capture")
+sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
 from flight_analyze import load  # noqa: E402