Fix deferred FK mismatch on savepoint release

[pedrocarlo]

Description

• Pager: updated release_savepoint and rollback_to_newest_savepoint in core/storage/pager.rs to locate and remove the most-recent Statement savepoint via rposition and update parent offsets correctly, and only truncate the subjournal when appropriate.
• FK generation: simplified parent-key UPDATE handling in core/translate/fkeys.rs by removing the NEW-key probe from emit_fk_parent_pk_change_counters and keeping NEW-key reconciliation in the REPLACE-specific path, avoiding over-decrements of the deferred-FK counter.
• Tests: added a deterministic reproducer release_root_named_savepoint_checks_deferred_fk in tests/fuzz/savepoint.rs that asserts parity with SQLite for releasing a root named savepoint that triggers deferred FK failures.

Motivation and context

• A differential fuzz seed revealed a parity failure with SQLite when releasing nested named savepoints that left statement-savepoints stacked beneath them, causing deferred foreign-key counters to be mishandled.
• The intent is to ensure statement savepoint cleanup and deferred-FK bookkeeping match SQLite semantics so release/rollback behavior is consistent.

Description of AI Usage

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[turso-bot]

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