Preserve structural sharing in update, intersect and leaf combine

[arthaud]

A Patricia tree is immutable and shared through Rc, so an operation whose result equals its input should return the input itself rather than rebuild it. merge_trees already did this; three other places did not, and because they allocated fresh nodes unconditionally they also defeated the Rc::ptr_eq fast paths that merge_trees, is_tree_subset_of and is_tree_leq rely on for sublinear behaviour on later operations. That matters here: fixpoint iteration merges near-identical environments over and over, and every lost identity makes the next round more expensive.

This mirrors what the C++ implementation does in PatriciaTreeCore.h.

• update_node_by_key now returns the existing branch when the rebuilt child is pointer-equal to the old one, rather than rebuilding the whole root-to-leaf path. The comment asking for exactly this has been there since the file was written; C++ does it in update_leaf_by_key.

• intersect_trees now returns s when both subtrees come back unchanged, matching intersect_trees in C++.

• The leaf combiner keeps the existing leaf when the combined value equals the value already stored, instead of always allocating a new one. This is the C++ update_leaf_internal behaviour, and it is what makes the two checks above fire at all: for a set every value is (), so previously every shared leaf was reallocated during a union or intersection and no subtree ever looked unchanged.

Comparing a value requires V: Eq, which is now a bound on get_leaf_combine_with_value_op_semantics, union_with and intersect_with. All three are pub(crate) and every caller already satisfies it, so no public signature changes. C++ has the same requirement, via Value::equals in its Value contract.

Measured with a counting global allocator on a 10,000 element PatriciaTreeSet<u32>, for operations whose result equals their input:

remove(absent key)          14 -> 0 allocations
intersect_with(superset) 19999 -> 0 allocations
union_with(subset)         927 -> 0 allocations

insert of an already present key still costs one rebuilt path; it allocates its leaf before consulting the tree, and fixing it means adding V: Eq to PatriciaTreeMap::upsert, which is a public signature. Left for a separate change.

Test plan: cargo test — 55 tests pass, including two new ones covering that no-op operations keep the root pointer and that a combine which does change a value still updates it. cargo clippy --all-targets emits the same set of warnings as main; cargo fmt --check reports no diff for this file.