fix(tdigest): validate deserialized state invariants

[tisonkun]

Summary

• reject unknown or mutually exclusive T-Digest serialization flags
• validate finite, ordered extrema and require non-empty images to store at least one value
• validate centroid ordering and range membership, plus buffered-value range membership
• apply equivalent structural checks to the compatibility formats

Performance

The validation loops deliberately accumulate the floating-point predicates and branch once after each payload. This keeps valid-state deserialization on a straight-line fast path.

Paired Divan runs against origin/main on the same machine:

Benchmark	origin/main	This PR	Change
deserialize_partial_groups (large)	102.2 µs	72.22 µs	-29.3%
deserialize_partial_groups (small)	19.2 µs	16.06 µs	-16.4%

Allocation counts and bytes were unchanged. The speedup comes from fusing the former per-value NaN and finite checks into the structural validation pass; the main purpose of this PR remains rejecting malformed states rather than promising a particular performance gain.

Validation

• cargo x prepare-testdata
• cargo x check
• cargo x lint
• cargo x test