fix(release): bump TOOL_VERSION to 0.26.0 — sidecar staleness was inert

TOOL_VERSION had been frozen at 0.21.1 for four releases despite its own
comment saying "Bump this constant and CHANGELOG.md together at release
time".  It is not cosmetic: backfill_sidecars.py decides whether to
regenerate with

    ver_ok = sidecar.source.tool_version >= event_file_io.TOOL_VERSION

so with the constant stuck at 0.21.1 and every sidecar stamped 0.21.1,
a backfill WITHOUT --force skipped the entire store.  That is precisely
the failure the check exists to prevent, and it means every sidecar
regenerated during the 0.26.0 decode work is stamped 0.21.1 while having
been produced by 0.26.0 code.

Verified: a non-force dry-run over the snapshot now reports
written=11603 skipped(uptodate)=0, where before it would have skipped
all 11,603.  Prod therefore does not need --force to pick up the decode
corrections — the version difference alone is enough.

Note the installed dist metadata reads 0.12.0, older than the constant,
so the best-effort "prefer installed metadata when newer" path correctly
defers to TOOL_VERSION.

Also bumps the README header, which still read v0.22.0.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HgTe8CamXAHcAmaQ6QNcog

README.md

@@ -1,4 +1,4 @@
-# seismo-relay  `v0.22.0`
+# seismo-relay  `v0.26.0`
 
 A ground-up replacement for **Blastware** — Instantel's aging Windows-only
 software for managing seismographs.  Supports both the **MiniMate Plus

minimateplus/event_file_io.py

@@ -50,7 +50,7 @@ SIDECAR_KIND   = "sfm.event"
 # bumped without a `pip install` re-run — leading to confusing stale
 # version stamps in sidecars.  Bump this constant and CHANGELOG.md
 # together at release time.
-TOOL_VERSION = "0.21.1"
+TOOL_VERSION = "0.26.0"
 
 try:
     # Best-effort: prefer the installed metadata when it's NEWER than the