fix(kb): report an impossible front-matter date instead of crashing - #66

kb.parse() only catches yaml.YAMLError, but PyYAML builds datetime.date objects for unquoted YYYY-MM-DD values. A day that doesn't exist (date: 2026-09-31, 2026-02-30, a swapped 2026-30-09) raises a plain ValueError from the constructor, so the error goes straight up to the user:

$ um kb check knowledge/techniques/t.md
Traceback (most recent call last):
  ...
  File ".../yaml/constructor.py", line 330, in construct_yaml_timestamp
    return datetime.date(year, month, day)
ValueError: day is out of range for month

Because notes() parses every file, one such note also crashes um kb search and um kb index for the whole knowledge base. check_note already has a friendly path for broken front matter (front matter is not valid YAML: ...), and the date check further down never gets a chance to run.

Change: catch ValueError next to yaml.YAMLError in parse(), so the note is reported as front matter is not valid YAML: day is out of range for month and the other notes still search and index.

Test: test_kb_impossible_date_is_reported_not_raised checks the note's failure message and runs search() and build_index() over a knowledge base containing it. It raises ValueError on main and passes with this change.

uv run --with pytest pytest -q tests   # 58 passed
./bin/um kb check --index               # PASS: 44 notes checked

Checklist

• uv run --with pytest pytest -q tests passes (code)
• no game files, extracted assets, decompiled code dumps or secrets
• nothing that cheats other players or bypasses anti-cheat / DRM / ownership checks
• authored by: @kevin9327