feat(table): variant predicate pushdown and reject variant partition source

[nssalian]

Adds predicate pushdown for shredded variant fields and rejects Variant as a partition
source. Closes #1016.

Changes

1. Extract expressions - extract(variant, path, type) referencing a field inside a variant column. RFC-9535 path parse (variant_path.go), bind against the shredded schema (variant_extract.go), cast to the target type (variant_cast.go). Follows Java Expressions.extract / PathUtil.parse / VariantExpressionUtil.castTo.

2. File pruning - inclusive/strict metrics evaluators use a shredded field's stored bounds for an extract term (table/evaluators.go); the strict path never prunes on extract. Bounds are split via arrow-go Metadata.SizeBytes()
   (table/internal/variant_bounds.go). Follows Java InclusiveMetricsEvaluator /StrictMetricsEvaluator.

3. Residual filtering (columnar) - surviving extract terms become synthetic derived columns evaluated through the existing arrow-go compute record-filter path, not row-by-row (scanTranslator in visitors.go, table/variant_residual.go,table/arrow_scanner.go).

4. Reject Variant partition source - addSpecFieldInternal (partitions.go) rejects a Variant source for any transform. Builds on #1206 (which added Variant to IdentityTransform's deny-list); this path does not consult CanTransform, so it closes the gap for sources added via AddPartitionFieldBySourceID.

Testing

• Unit tests for path parse, cast, extract binding, residual eval, metrics pruning, and partition rejection; end-to-end shredded-write + pushdown test (table/variant_shredded_write_test.go).