fix(isolated-declarations): preserve variable declarations when type with same name is referenced in implements clause (#16328)

- [x] Understand the issue: variable declarations with shadowed type
names are dropped when the type is referenced in implements clause
- [x] Identify root cause: `add_reference` and `resolve_references` were
overwriting flags instead of combining them
- [x] Implement fix: use bitwise OR to combine `KindFlags` when adding
references and merging scopes
- [x] Add test case to fixtures (extended shadowed.ts with issue #10996
test case)
- [x] Run tests and update snapshots
- [x] Run code review 
- [x] Run CodeQL security check (no issues found)
- [x] Verify fix manually with original issue example
- [x] Reverted .gitignore change per reviewer feedback

## Summary

This PR fixes issue #10996 where variable declarations with shadowed
type names were incorrectly dropped when the type was referenced in a
class `implements` clause.

### Problem
When a variable name is shadowed by a type alias and that type is
referenced in `implements`, the variable declaration was missing from
`.d.ts` output.

### Root Cause
`ScopeTree` used `HashMap::insert()` which overwrites reference flags.
When a name had both `KindFlags::Value` (from `typeof Test2`) and
`KindFlags::Type` (from `implements Thing<Test2>`), only the last one
was preserved.

### Fix
Changed `add_reference()` and `resolve_references()` to combine flags
using bitwise OR (`|=`) instead of overwriting.

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<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>[isolatedDeclarations] variable emit is dropped if the
variable is shadowed as a type name and the type is part of a class
implements interface</issue_title>
> <issue_description># Repo
> 
> ```ts
> export interface Thing<T> {}
> 
> /** WORKS */
> 
> const Test1 = 'test1' as const;
> export type Test1 = typeof Test1;
> export class Class1 {
>   readonly id: 'test1' = Test1;
> }
> 
> /** DOES NOT WORK */
> 
> const Test2 = 'test2' as const;
> export type Test2 = typeof Test2;
> export class Class2 implements Thing<Test2> {
>   readonly id: 'test2' = Test2;
> }
> ```
> 
> # Expected
> 
> ```ts
> export interface Thing<T> {
> }
> /** WORKS */
> declare const Test1: "test1";
> export type Test1 = typeof Test1;
> export declare class Class1 {
>     readonly id: 'test1';
> }
> /** DOES NOT WORK */
> declare const Test2: "test2";
> export type Test2 = typeof Test2;
> export declare class Class2 implements Thing<Test2> {
>     readonly id: 'test2';
> }
> export {};
> ```
> [ts
playground](https://www.typescriptlang.org/play/?#code/KYDwDg9gTgLgBASwHY2FAZgQwMbDgFQAtkBzAHnwD44BvAXwCgGB6AKlbgHUB5AJQGkAynFbMm2CEgDO8fMBkBGOAF44AclSK1cTFLgTpMANxxmzODGJ6SwGHuABbBDFQATBqEiwLATzB45RRVffwh0AnkYBSMPcGh4bAAbXT0AYWSpKSUaBjg4KGBMV0lEn0RXAC51TSjtVUComMYWdjgAEW4AUWEAOW58Lj5+ETEGAxkImQAmYI1Iqe1dfUkZEzM4Yvk4JAh4G3hHZzdYr3gYPwD54PPQ8IapmM94-Qy015mEBzBEx2AUPSIpAo82oOTyBSKJTKCEq1XmdUmMAeDGaQA)
> 
> # Actual
> 
> ```ts
> export interface Thing<T> {}
> /** WORKS */
> declare const Test1: "test1";
> export type Test1 = typeof Test1;
> export declare class Class1 {
> 	readonly id: "test1";
> }
> export type Test2 = typeof Test2;
> export declare class Class2 implements Thing<Test2> {
> 	readonly id: "test2";
> }
> export {};
> ```
> [oxc
playground](https://playground.oxc.rs/#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)
> 
> Note the missing emit of `declare const Test2:
"test2";`</issue_description>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>

- Fixes #16314

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Co-authored-by: Dunqing <[email protected]>

Author: Copilot

crates/oxc_isolated_declarations/src/scope.rs

@@ -66,7 +66,7 @@ impl<'a> ScopeTree<'a> {
 
     fn add_reference(&mut self, name: Atom<'a>, flags: KindFlags) {
         let scope = self.levels.last_mut().unwrap();
-        scope.references.insert(name, flags);
+        scope.references.entry(name).and_modify(|f| *f |= flags).or_insert(flags);
     }
 
     /// Resolve references in the current scope, and propagate unresolved ones.
@@ -84,7 +84,10 @@ impl<'a> ScopeTree<'a> {
         });
 
         // Merge unresolved references to the parent scope.
-        self.levels.last_mut().unwrap().references.extend(current_references);
+        let parent_scope = self.levels.last_mut().unwrap();
+        for (name, flags) in current_references {
+            parent_scope.references.entry(name).and_modify(|f| *f |= flags).or_insert(flags);
+        }
     }
 }
 

crates/oxc_isolated_declarations/tests/fixtures/shadowed.ts

@@ -13,4 +13,21 @@ type Module = () => void;
 namespace Module {
 	export const x = 1;
 }
-export type ModuleType = Module;
+export type ModuleType = Module;
+
+// https://github.com/oxc-project/oxc/issues/10996
+// Variable emit should not be dropped when variable is shadowed as a type name
+// and the type is part of a class implements interface
+export interface Thing<T> {}
+
+const Test1 = 'test1' as const;
+export type Test1 = typeof Test1;
+export class Class1 {
+  readonly id: 'test1' = Test1;
+}
+
+const Test2 = 'test2' as const;
+export type Test2 = typeof Test2;
+export class Class2 implements Thing<Test2> {
+  readonly id: 'test2' = Test2;
+}

crates/oxc_isolated_declarations/tests/snapshots/shadowed.snap

@@ -17,4 +17,18 @@ declare namespace Module {
 	const x = 1;
 }
 export type ModuleType = Module;
+// https://github.com/oxc-project/oxc/issues/10996
+// Variable emit should not be dropped when variable is shadowed as a type name
+// and the type is part of a class implements interface
+export interface Thing<T> {}
+declare const Test1: "test1";
+export type Test1 = typeof Test1;
+export declare class Class1 {
+	readonly id: "test1";
+}
+declare const Test2: "test2";
+export type Test2 = typeof Test2;
+export declare class Class2 implements Thing<Test2> {
+	readonly id: "test2";
+}
 export {};