feat: implement gloss-drill org-drill export

Two public commands plus a small helper. `gloss-drill-export-all'
walks `gloss-file' via `org-map-entries' and adds `:drill:' tag and
`DRILL_CARD_TYPE: twosided' property to every top-level entry.
Membership and equality guards make the operation idempotent: re-running
adds nothing and writes nothing. `gloss-drill-untag-all' is the
reverse, and intentionally does not require `org-drill' to be installed
(the user might be cleaning up after uninstalling).

The walking logic factors into a single private helper
`gloss-drill--map-entries' that handles file open, modtime
verification, org-mode activation, the level-1 filter, and a
write-only-if-modified save. Both public commands compose it with their
respective per-entry mutators.

`org-drill' presence is checked with `featurep' before any walk so the
file is never touched when the dep is missing. The user-error message
includes the install command.

Folds in a small fix to the idempotency test helper: the original used
`throw' from inside `org-map-entries' but did not return the count to
the caller. Switched to `catch' / `throw' with the count as the throw
value.

98 tests pass in 0.24s — 88 prior plus 10 new across the four scenarios
named in the design doc (tags-untagged, skips-already-tagged,
no-orgdrill-installed, untag-all).

0 files changed, 0 insertions, 0 deletions