refactor(wcs)!: Drop the WCS keyword table, invalidate the cached WCS on any header write

[icweaver]

setindex! on an AstroImage header key previously consulted a ~250-line WCS_HEADERS_TEMPLATES table (expanded into an exact-match Set) to decide whether to mark the cached WCSTransform stale. Such a filter can't be complete. WCS keywords span Papers I–IV plus legacy conventions, with unbounded axis indices and A–Z alternate-system suffixes, and the table had real holes: the expansion only covered axis indices 1–4 and lowercase alternate suffixes, so e.g. img["CRPIX5"] = 3.0 or img["CRPIX1A"] = 3.0 silently left a stale transform in use.

Now any header write marks the cached WCS stale, and it is lazily re-parsed on the next wcs() access. Re-parsing is cheap and header writes are rare, so the extra invalidations cost effectively nothing, while false negatives are gone by construction. The staleness regression test now covers the previously-missed key classes.

Breaking only in the internal sense: the unexported WCS_HEADERS_TEMPLATES/WCS_HEADERS constants are gone, and cache invalidation is now unconditional rather than keyword-filtered.

Split out from #119

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.95%. Comparing base (bb6bcdf) to head (f52d8c3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #126      +/-   ##
==========================================
+ Coverage   61.13%   61.95%   +0.82%     
==========================================
  Files           9        9              
  Lines        1078     1062      -16     
==========================================
- Hits          659      658       -1     
+ Misses        419      404      -15     

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