[TRACKING] Enable cross-version deserialization (RFC 0002)

[andrewherren]

Tracking issue for RFC 0002: https://github.com/StochasticTree/rfcs/blob/main/0002-cross-version-deserialization.md

Implements intra-platform (R→R, Python→Python) cross-version deserialization for BART and BCF models. Cross-platform (R↔Python) deserialization is explicitly out of scope and will be addressed in a future RFC (main blocker: covariate preprocessor schema divergence).

Sub-issues

• #317 — Add stochtree_version stamp to JSON output (R + Python)
• #318 — Version inference helper for legacy JSONs
• #319 — Robust BART from_json: safe defaults + warnings for missing fields
• #320 — Robust BCF from_json: safe defaults + warnings for missing fields
• #321 — Fix BCF field-name mismatches between R and Python (prerequisite for future cross-platform work)
• #322 — Snapshot tests and backward-compat deserialization tests