refactor: split pushdown and residual predicates in scan plan

[belveryin]

Summary

• keep source pushdown (pushdown_predicate) separate from post-merge filtering (residual_predicate)
• compute residual specifically for non-SST sources so only non-pushdownable clauses are evaluated in PackageStream
• remove post-merge evaluation of pushdown_predicate; evaluate residual only
• apply early merge limit whenever residual is empty

Tests

• cargo test
• cargo clippy -- -D warnings
• targeted scan tests added/updated in src/db/tests/core/scan.rs

[ethe]

StartsWith range pruning is not sound for non-SST scans. next_prefix_string can return None on Unicode boundary cases (when char+1 is not a valid scalar), which degrades pruning to a lower-bound-only key range. In non-SST paths, StartsWith is treated as fully pushdown (residual = None), so false positives are never re-filtered.

[ethe]

Composite-key + Expr::True incorrectly forces residual filtering and disables early merge limit. split_predicate_for_non_sst returns residual: Some(predicate.clone()) for multi-column keys before handling Expr::True, which propagates to needs_post_filter = true and turns off early merge limit.

[ethe]

StartsWith can be silently dropped for non-string primary keys on non-SSTscans.
In split_predicate_for_non_sst_inner, Expr::StartsWith is treated as fullpushdown when the column matches and next_prefix_string(prefix) exists (src/db/scan.rs:1215), but this path does not verify the key type.However, key-range derivation only supports StartsWith for string keys (src/query/scan.rs:523). For non-string keys, no key range is produced, and becauseresidual is None (src/db/scan.rs:1232, consumed at src/db/scan.rs:226),the predicate is not evaluated at all. This is a correctness issue

[belveryin]

StartsWith can be silently dropped for non-string primary keys on non-SSTscans. In split_predicate_for_non_sst_inner, Expr::StartsWith is treated as fullpushdown when the column matches and next_prefix_string(prefix) exists (src/db/scan.rs:1215), but this path does not verify the key type.However, key-range derivation only supports StartsWith for string keys (src/query/scan.rs:523). For non-string keys, no key range is produced, and becauseresidual is None (src/db/scan.rs:1232, consumed at src/db/scan.rs:226),the predicate is not evaluated at all. This is a correctness issue

Good catch! Though the phrasing is not completely accurate: this path wasn’t silently dropping the predicate because residual evaluation was still preserving it end-to-end.

This uncovered a real inconsistency, though: non-SST StartsWith pushdown did not check primary-key type while key-range derivation is string-only. The last commit makes non-SST StartsWith type-aware and added a regression test, so non-string PKs keep StartsWith as residual instead of being treated as full pushdown.