Thread PageMut lifetime through access guards and builders

[cberner]

Motivation

• Ensure the logical lifetime of mutable pages is represented in types so AccessGuard/AccessGuardMut lifetimes correctly track the PageMut they reference.
• Replace a standalone _lifetime: PhantomData<&'a ()> on guards with a PageMut<'a> so lifetimes are tied to the actual mutable page handle inside EitherPage.
• Fix several related lifetime/borrowing issues by consistently threading the PageMut lifetime through functions that construct or return mutable pages.

Description

• Added a lifetime parameter to PageMut and a PhantomData field to carry that lifetime in src/tree_store/page_store/base.rs.
• Made EitherPage generic over the page lifetime and stored PageMut<'a> inside it, and updated AccessGuard<'a, V> to hold EitherPage<'a> in src/tree_store/btree_base.rs.
• Propagated the PageMut<'a> lifetime into AccessGuardMut<'a, V> and AccessGuardMutInPlace<'a, V> and adjusted their constructors and fields accordingly in src/tree_store/btree_base.rs.
• Kept mutable page allocation and retrieval APIs returning PageMut<'static> so mutable handles remain moveable without tying them to a borrow of TransactionalMemory, updating src/tree_store/page_store/page_manager.rs.
• Updated B-tree helpers and mutation callbacks to use the new PageMut typing (notably dirty_leaf_visitor, get_mut_helper, and insert_inplace_helper) in src/tree_store/btree.rs and src/tree_store/btree_mutator.rs.
• Aligned builder return types to PageMut<'static> for leaf/branch builders to match the new model in src/tree_store/btree_base.rs.
• Files changed: src/tree_store/page_store/base.rs, src/tree_store/page_store/page_manager.rs, src/tree_store/btree_base.rs, src/tree_store/btree.rs, and src/tree_store/btree_mutator.rs.

Testing

• Ran the full test suite with cargo test -q and all automated tests completed successfully.

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Codex Task