Add mutable IMap<K, V> stock implementation to windows-collections

[Copilot]

Adds a mutable IMap<K, V> stock implementation to windows-collections, mirroring the mutable IVector<T> added in #4322.

• src/map.rs: StockMap<K, V> backed by RwLock<BTreeMap<K::Default, V::Default>>, implementing IMap<K, V> + IIterable<IKeyValuePair<K, V>>. Insert returns true when replacing an existing key; Remove returns E_BOUNDS for missing keys. GetView returns a point-in-time snapshot.
• StockMapIterator: holds a ComObject<StockMap<K, V>> reference and an AtomicUsize current index (same pattern as StockVectorIterator), reading directly from the owner's RwLock<BTreeMap> on each call — no upfront copy at First() time.
• src/key_value_pair.rs: shared StockKeyValuePair<K, V> type extracted into its own module and used by both map.rs and map_view.rs, eliminating the previous duplication.
• From<BTreeMap<K::Default, V::Default>> impl for IMap<K, V>.
• Tests: covers read, mutate, iterator, GetView snapshot isolation, and HSTRING keys.

let m = IMap::<HSTRING, i32>::from(BTreeMap::new());
m.Insert(h!("key"), 42)?;
assert_eq!(m.Lookup(h!("key"))?, 42);
assert!(m.Insert(h!("key"), 99)?); // true = replaced

let view = m.GetView()?;
m.Insert(h!("other"), 1)?;
assert_eq!(view.Size()?, 1); // snapshot unaffected

[kennykerr]

@copilot The Vector implementation avoids the copy in its IIterable implementation. Is there a reason that is not possible here?

[Copilot]

Fixed in cbd0426. StockMapIterator now holds owner: ComObject<StockMap<K, V>> and an AtomicUsize current index — the same pattern as StockVectorIterator. First() no longer copies the map entries; iterator methods read directly from the owner's RwLock<BTreeMap> using map.iter().nth(current) / map.iter().skip(current).

[kennykerr]

@copilot this looks like the same as the one used in map_view.rs - can they be shared?

[Copilot]

Done in c18cd21. The type is now defined once in a new key_value_pair.rs module as StockKeyValuePair, and both map.rs and map_view.rs reference it via super::key_value_pair::StockKeyValuePair. The duplicate StockMapKeyValuePair in map.rs and the old StockKeyValuePair in map_view.rs have been removed.