fix(index): make vector append segment-set-native

[Xuanwo]

Closes #6398.

A logical vector index can contain immutable physical segments trained with different IVF and quantizer models. The append path currently validates the entire segment set before selecting work and then tries to encode unindexed fragments as though every segment shared one model. This makes append fail on valid heterogeneous segment sets.

This makes append segment-set-native. Append builds one new physical segment over only unindexed fragments using the complete model of the deterministic manifest suffix segment, while preserving all existing segment metadata and fragment coverage. Explicit merge selects its requested suffix before compatibility validation, and steady-state rebalance continues to rewrite only one segment. Explicit retrain remains the operation that source-rebuilds and unifies models.

The compatibility guard remains fail-closed and now applies to legacy and V3 storage, including IVF, quantizer/codebook/rotation, metric, dimension, and index-type metadata. This intentionally leaves distributed merge planning in #7730 out of scope.

[codecov]

Codecov Report

❌ Patch coverage is 79.82301% with 114 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance/src/index/vector.rs	33.33%	48 Missing ⚠️
rust/lance/src/index/append.rs	86.62%	26 Missing and 18 partials ⚠️
rust/lance/src/index/vector/ivf.rs	69.01%	20 Missing and 2 partials ⚠️

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[BubbleCal]

I left three inline comments on compatibility, logical-index validation, and the ANN regression test.