API: Rewrite string truncate equality predicates onto the source column

[wombatu-kun]

What

Resolves the long-standing TODO: translate truncate(col) == value to startsWith(value) in UnboundPredicate.bindLiteralOperation. When the term of an EQ/NOT_EQ predicate is a string truncate[W] transform, binding now produces an exactly-equivalent predicate on the untransformed source column.

The equivalence depends on the literal length vs. the truncate width W:

Condition	truncate[W](col) == v	truncate[W](col) != v
len(v) > W	alwaysFalse()	alwaysTrue()
len(v) == W	col STARTS_WITH v	col NOT_STARTS_WITH v
len(v) < W	col == v	col != v

Integer/long/decimal/binary truncate and all other operators are intentionally left unchanged — they have no exact source-column equivalence.

Why

This rewrite is already assumed by the rest of the engine. InclusiveMetricsEvaluator.startsWith() returns ROWS_MIGHT_MATCH for non-identity transform terms with the explicit comment "truncate must be rewritten in binding". Until now the binder never performed that rewrite for equality, so truncate(col) == v kept an opaque BoundTransform term and metrics/dictionary/partition pruning could not use the column. After this change such predicates prune correctly (e.g. equal(truncate("str",3),"xyz") against bounds ["abc","abe"] now skips the file instead of reading it).

Implementation notes

• The < / == / > width decision is centralized in a single Truncate.lengthRewrite helper, shared by predicate binding and by TruncateString.project / projectStrict, so the two paths cannot diverge.
• Removing the BoundTransform term would otherwise defeat ProjectionUtil.projectTransformPredicate (which matches partition transforms by toString()) and collapse strict projection to False. To preserve the previous precision, TruncateString now projects EXACT-length (len(v) < W) EQ/NOT_EQ predicates directly onto the partition value — provably the same result the old transform-term path produced, since truncate[W](x) == v ⟺ x == v when len(v) < W.
• New public API is additive only (Transforms.StringTruncateRewrite enum + Transforms.stringTruncateRewrite); revapi passes with no accepted-breaks entry.

Testing

• TestPredicateBinding: all three length classes for EQ/NOT_EQ, empty-string literal, plus negatives (non-string truncate, non-truncate transforms, other operators unchanged).
• TestStartsWith: runtime-equivalence check via Evaluator, including the EXACT-vs-prefix distinction.
• TestInclusiveMetricsEvaluatorWithTransforms: a pruning case that now prunes where it previously could not.
• Full :iceberg-api:test and :iceberg-core:test, all transform/projection/residual regression suites, spotlessCheck, and :iceberg-api:revapi pass.

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