perf(transformer): pre-size statement vecs in TS enum & namespace lowering (#23516)

## What

Two zero-capacity arena `Vec`s in the TypeScript transformer were filled
in known-length loops and grew via reallocation. Pre-size them —
mirroring the JSX-props pre-size in #23466:

- **`transform_ts_enum_members`**: emits exactly `members.len() + 1`
statements (one per member + the final `return`) →
`vec_with_capacity(members.len() + 1)` (an **exact** capacity).
- **TS namespace `enter_program`**: rebuilds the program body; pre-size
to `program.body.len()` (an estimate — most statements pass through
1:1).

## Δ — fewer reallocations on TS fixtures

`allocs_transformer.snap`, reallocation count:

| fixture | before | after |
| --- | --- | --- |
| checker.ts | 64 | **5** |
| kitchen-sink.tsx | 304 | **280** |
| App.tsx | 25 | **17** |
| binder.ts | 6 | **0** |

(allocation **count** and **bytes** are unchanged — this removes the
intermediate growth reallocations.)

## Why it is behavior-preserving

Capacity-only: `vec_with_capacity` differs from `vec` solely in initial
capacity; element type, push order, and contents are identical, so the
produced AST is byte-identical. `oxc_transformer` tests pass.

Prepared with AI assistance.

crates/oxc_transformer/src/typescript/enum.rs

@@ -310,7 +310,9 @@ impl<'a> TypeScriptEnum {
     ) -> ArenaVec<'a, Statement<'a>> {
         let ast = ctx.ast;
 
-        let mut statements = ast.vec();
+        // Each member pushes exactly one statement, plus a final `return` statement,
+        // so the length is known up front — pre-size to avoid growth reallocations.
+        let mut statements = ast.vec_with_capacity(members.len() + 1);
 
         // If enum number has no initializer, its value will be the previous member value + 1,
         // if it's the first member, it will be `0`.

crates/oxc_transformer/src/typescript/namespace.rs

@@ -37,8 +37,10 @@ impl<'a> Traverse<'a, TransformState<'a>> for TypeScriptNamespace {
 
         // Recreate the statements vec for memory efficiency.
         // Inserting the `let` declaration multiple times will reallocate the whole statements vec
-        // every time a namespace declaration is encountered.
-        let mut new_stmts = ctx.ast.vec();
+        // every time a namespace declaration is encountered. Pre-size to the current body length
+        // (a lower bound — namespaces expand) to avoid the growth reallocations for the common
+        // pass-through statements.
+        let mut new_stmts = ctx.ast.vec_with_capacity(program.body.len());
 
         for stmt in program.body.take_in(ctx.ast) {
             match stmt {

tasks/track_memory_allocations/allocs_transformer.snap

@@ -1,8 +1,8 @@
 File                       | File size      || Sys allocs     | Sys reallocs   || Arena allocs   | Arena reallocs 
 ---------------------------------------------------------------------------------------------------------------------------
-checker.ts                 | 2.92 MB        ||             20 |              0 ||           4400 |             64
+checker.ts                 | 2.92 MB        ||             20 |              0 ||           4400 |              5
 
-App.tsx                    | 415.34 kB      ||             25 |              2 ||            719 |             25
+App.tsx                    | 415.34 kB      ||             25 |              2 ||            719 |             17
 
 RadixUIAdoptionSection.jsx | 2.52 kB        ||             21 |              2 ||            259 |             13
 
@@ -10,7 +10,7 @@ pdf.mjs                    | 567.30 kB      ||             15 |              0 |
 
 antd.js                    | 6.69 MB        ||             15 |              0 ||              2 |              0
 
-binder.ts                  | 193.08 kB      ||             17 |              0 ||            379 |              6
+binder.ts                  | 193.08 kB      ||             17 |              0 ||            379 |              0
 
-kitchen-sink.tsx           | 732.90 kB      ||            184 |              3 ||           6690 |            304
+kitchen-sink.tsx           | 732.90 kB      ||            184 |              3 ||           6690 |            280