docs: add ContractProbe.md design — dry-run capability check epic

Forward-looking design for cppmega-mlx-728 (Contract Probe), drafted
in the same style as VisualBuilderSpec.md / ModelBuildSpec.md /
ParallelismSpec.md.

Outlines a new read-only cppmega_v4.probe package that:
- introspects tokenizer + parquet capabilities (Stage A)
- statically tables per-brick and per-loss data requirements
- runs a deterministic solver that emits ContractProbeReport with
  explicit alternatives + JSON-Patch diffs (Stage B)
- exposes a stable JSON API + CLI hand-off for the GUI epic (Stage C)

Explicitly NOT in scope: auto-apply alternatives, LLM-generated
suggestions, multi-shard aggregation, probe-time training. Probe
proposes; human decides; GUI emits diff.

Implementation tickets cppmega-mlx-728.1/.2/.3 remain open; this
commit lands the contract before code.

Author: apstenku123