fix conflit for 2145

[peter-jerry-ye]

Try to solve conflicts for #2145

@Asterless See if this branch is what you want

[peter-jerry-ye-code-review]

Potential memory leak in intersection implementation for Branch nodes

Category
Correctness
Code Snippet
Branch(sa1.intersection(sa2, fn(m1, m2) { m1.intersection(m2) }))
Recommendation
Add a size check and return Empty for empty results like in HashSet implementation:

let res = sa1.intersection(sa2, fn(m1, m2) { m1.intersection(m2) })
if res.size() == 0 { Empty } else { Branch(res) }```
**Reasoning**
The HashSet implementation includes this optimization to avoid creating unnecessary Branch nodes for empty intersections. The HashMap should follow the same pattern for consistency and memory efficiency.

</details>
<details>

<summary> Duplicated folding pattern in set operations </summary>

**Category**
Maintainability
**Code Snippet**
.fold(init=Empty, fn(m, kv) {
  match other.get(kv.0) {
    Some(v2) => m.add(kv.0, f(kv.0, kv.1, v2))
    None => m
  }
})
**Recommendation**
Extract common folding pattern into a helper function:
```moonbit
fn fold_matching[K,V,R](self: T[K,V], other: T[K,V], init: R, f: (R,K,V,V) -> R) -> R```
**Reasoning**
Similar folding patterns appear in union_with, intersection_with and difference implementations. Extracting to a helper would reduce code duplication and make the operations easier to maintain.

</details>
<details>

<summary> Unnecessary traversal in difference operation </summary>

**Category**
Performance
**Code Snippet**
difference[K : Eq + Hash](self : T[K, V], other : T[K, V]) -> T[K, V]
**Recommendation**
For Branch case, compute difference directly on matching subtrees:
```moonbit
Branch(sa1.difference(sa2, fn(m1, m2) { m1.difference(m2) }))```
**Reasoning**
The current implementation falls back to full iteration for Branch nodes instead of operating directly on matching subtrees, which is less efficient for large maps with similar structure.

</details>

[coveralls]

Pull Request Test Coverage Report for Build 6859

Details

• 40 of 81 (49.38%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.5%) to 90.763%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
immut/internal/sparse_array/sparse_array.mbt	5	6	83.33%
immut/hashset/HAMT.mbt	7	22	31.82%
immut/hashmap/HAMT.mbt	28	53	52.83%

Totals
Change from base Build 6851:	-0.5%
Covered Lines:	6377
Relevant Lines:	7026

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💛 - Coveralls