feat(flashcard): support multi-tag drill headings + --tag-filter / --guid-salt

Craig wanted a curated "DeepSat Fundamentals" deck: the ~100 most fundamental cards out of the 465-card deepsat.org deck, marked with a second org tag (:fundamental:) so they stay grep-able in the source. Both flashcard-to-anki.py and flashcard-stats.py keyed a card on a heading ending in exactly :drill:, so adding any second tag turned the heading into :fundamental:drill:, which that anchor didn't match. The 100 tagged cards would have silently dropped from the full-deck apkg and been undercounted by stats. That's the passing-gate-skips-your-file failure.

CARD_RE now matches a trailing org tag block, and a heading is a card when drill is among its tags. The other tags ride along as Anki tags next to the section tag. Card bodies are bounded by any L1/L2 heading (HEADING_RE) rather than only a section or a drill heading. flashcard-to-anki.py gains --tag-filter <tag> (emit only cards carrying that org tag, the subset deck) and --guid-salt <s> (a distinct GUID space so the subset imports non-empty instead of colliding with the full deck on GUID and importing empty). No salt is unchanged, so the existing deck's GUIDs and SRS state are untouched. flashcard-stats.py gets the same CARD_RE/HEADING_RE broadening plus a drill-membership guard.

This is the rulesets-canonical reconcile of a stopgap the work project ran locally. I re-derived it against the current canonical rather than copying the preserved files, which predate the #+TITLE deck-name fix (060a938) and would have reverted it.

parse()'s third element is now the Anki-tag list rather than a single tag string. Only build() and the tests consumed it. The existing parse tests move to the list contract, and new tests cover multi-tag parsing, --tag-filter, the guid salt, and the stats count. A bats case guards that a :fundamental:drill: card still counts. The full deck's real 465/100 check needs the work deck and isn't runnable here. Verified end to end instead on a synthetic multi-tag deck (2 cards, --tag-filter fundamental returns 1) through the real genanki path.

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