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>

docs/re/structures/regn-map-grid.md

@@ -379,7 +379,7 @@ nothing rises above chance.
 
 ❔ **What would actually settle it** is the code — find what reads a `REGN`
 object in the executable and watch which fields it dereferences. That is static
-PE work (`/work/*.pe`, offset = VA − 0x82000000) of the same kind that cracked
+PE work (the flat `.pe` in the project root, offset = VA − 0x82000000) of the same kind that cracked
 the `.slb` packing phase, and it is the honest next step rather than a
 twenty-first correlation.
 ---