chore(release): 5.50.0

[dgunning]

Summary

Release 5.50.0. Version bump + CHANGELOG fold; no code changes.

Six entries, all merged to main since 5.49.0:

• Fixed: data.sec.gov cache rules never matching (fix(cache): cache rules never matched data.sec.gov, or a mirror's own host #989); FilingSGML.html() UnicodeDecodeError on PDF primaries, raw-XML text() on non-ownership XML forms (with the 1.5-hour NPORT pathology), pre-1997 SGML dialect markup leaking into text() (fix(sgml): classify a document before decoding it #1047); truncated submissions now raise instead of silently parsing as zero documents, plus PUBLIC DOCUMENT COUNT cross-check (fix(sgml): raise on truncated submissions, cross-check the header's document count #1049)
• Performance: slotted table cell classes + gated original_html retention — 1.51GB → 1.34GB peak on the profiled ABS-15G (perf(documents): slot the table cell classes, gate original_html retention #1048)

Minor bump per the runbook: a perf change plus user-visible behaviour changes.

Fold check done: 6 - ** bullets in the dated section, [Unreleased] empty, nothing stranded in published sections (verified against git log e8ea6689..origin/main).

Post-merge (runbook steps 3–5)

Build from the merged commit in a throwaway worktree, verify_wheel.py 5.50.0, twine check then upload from a fresh venv with twine>=6.1, then gh release create v5.50.0 --target main.

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