Preserve incompatible and drawing-only board data

[ojowwalker77]

What changed

• adds an explicitly versioned board payload while retaining legacy array decoding
• preserves unknown future card records across edits and autosaves
• write-protects corrupt, future-version, and unknown-only boards
• writes private content-addressed recovery copies and verifies existing copies
• provides a persistent recovery warning with Show Recovery and Duplicate to Edit actions
• derives pruning from durable payload semantics so every visual/structured element counts as content

Root cause

Decode failures fell back to a text card that later autosave/navigation/quit paths could write over the original bytes. Blankness was also derived only from the lossy text mirror, so unnamed drawing-only boards could be pruned.

Validation

• swift test --filter BoardPersistenceTests — 15 passed
• ./script/check — release build plus 184 tests passed
• ./script/verify — release bundle identity/update/icon verification passed
• git diff --cached --check — passed
• staged Splus review — 0 must-fix, 0 concern; 44/44 changed-export audit complete

The unrelated local opencode.json deletion is not included.

Closes #102
Closes #97