A language folder falls back per field all the way down, or is refused

[maximsan]

Why

Found auditing #605. The overlay merged one level deep, so the per-field
fallback it promises stopped at the top of an entry — and most of the course's
prose sits below that. No ticket; the audit is in the conversation that
produced this.

Watch for

• The bug was silent deletion, not a fallback gap. A folder translating a
  help entry's steps replaced the whole list: three English steps became one
  Polish step, with no error. A lesson with three of nine cards translated lost
  six cards of teaching. Every bank carries nested prose — lessons.cards,
  card_kind_help.steps, mini_games.steps, mini_game_content.rounds,
  visual_guides.notes.
• Three things are now refused that were silently accepted, ruled by the
  owner: a list shorter than the master's, a field the master has no slot for
  (a misspelt key left the real one in English), and one id translated twice
  (whichever came last won). Each names the entry and the field to redraft.
• Marks are stripped at every depth now, not only the record's top level,
  since ADR-0026 marks a piece of text wherever it sits.
• A record broken by the folder names the folder. loadBank reported every
  parse failure against the English master, which is not where the bad field
  came from once an overlay is in play.
• Unreachable today. English ships alone, so no folder exists to hit any of
  this. It becomes reachable the day one lands — and Draft a language folder from the English master #603, the tool meant to
  guarantee a folder is complete, is not built.

Not here

Mark granularity still mismatches ADR-0026. Marks are keyed by top-level
field, so a lesson's only key is cards and one fingerprint covers all of
them — fix a typo in card three and every card re-opens for retranslation,
which is what the ADR's "per piece of text, not per entry" exists to prevent.
That is the drafting tool's shape to settle (#603), not the loader's.

Checks

The gates, all green locally: format, analyze, the metrics gate, the comment
cap on the touched files, check_changelog.sh, and the full suite at 3236
passing. Seven new overlay tests and two loader tests; the nine existing
overlay tests pass unchanged. Both defects were reproduced against the real
card_kind_help content before the fix.