devel/datamine: runtime data mining tool

[Alistair-Afton]

Summary

• Adds devel/datamine, a developer tool for observing live game state when writing scripts or reverse-engineering structures.
• watch [-n name] [-i frames] <expr>: polls a Lua expression each frame and prints old -> new to the DFHack console when it changes. Watches persist across invocations (script env), run via repeat-util, and are cleared on world unload.
• snap <name> <expr> / diff <name>: records a deep snapshot of every field reachable from the expression's result, then reports which paths changed -- the classic "do X, see what moved" workflow.
• find <expr> <substring>: prints paths of fields whose name or value matches.
• All walks are depth-capped (5), per-container capped (200 elements), leaf-capped (20000), cycle-safe via object addresses, and every field access is pcall-guarded.
• Nothing runs unless a command is invoked; nothing is enabled by default.

Test plan

• watch registers, polls per-frame even while paused, prints on change, unwatch/unwatch all stop it
• snap on unit.name (19 fields), unit.status (2638 fields), units.active (20011 fields, leaf-capped)
• diff catches a written field (.nickname -> ZZTEST) and a stress write (.souls[0].personality.stress: 13486 -> 99999)
• find matches field-name substrings and value substrings, descends into containers
• error paths: bad expression at watch time, nonexistent watch name, unknown subcommand
• devel/datamine help renders the embedded doc