perf(codegen): pre-allocate sourcemap builder vectors (#27230)

## Summary

Pre-allocate the `SourcemapBuilder` `tokens`, `names`, and line-offset `lines` vectors from source length. This avoids repeated growth and memcpy work while preserving the existing `shrink_to_fit` behavior for final memory usage.

## Validation

- `cargo fmt --all -- --check` passed.
- `cargo test -p oxc_codegen`: all 12 sourcemap unit tests passed; the broader integration run had 172 passing tests and one unrelated snapshot mismatch because the local Node.js is v22.23.2 while the snapshot records v26.5.0.
- CodSpeed CPU Simulation comparison: [base vs head](https://app.codspeed.io/oxc-project/oxc/runs/compare/6abd3aefcdcfa72af53b6cc4..6abd3b51836fe9d134c5094b)
  - `codegen[kitchen-sink.tsx]`: 25.7 ms → 22.1 ms (+16.5%)
  - `codegen[react.development.js]`: 1.9 ms → 1.7 ms (+13.16%)
  - `codegen[App.tsx]`: 10.9 ms → 9.7 ms (+11.69%)
  - `codegen[binder.ts]`: 4.0 ms → 3.6 ms (+11.44%)
  - 8 improvements, 12 unchanged, and no regressions reported.

Wall-time validation was attempted locally but could not start because the sandbox cannot install the required `linux-tools` packages. The simulation run completed and uploaded successfully.

crates/oxc_codegen/src/sourcemap_builder.rs

@@ -66,12 +66,14 @@ pub struct SourcemapBuilder<'a> {
 impl<'a> SourcemapBuilder<'a> {
     pub fn new(path: &Path, source_text: &'a str) -> Self {
         let line_offset_tables = Self::generate_line_offset_tables(source_text);
+        // Estimate mapping and name counts from source length to reduce reallocations.
+        let source_len = source_text.len();
         Self {
             source_name: path.to_string_lossy().into_owned(),
             original_source: source_text,
-            names: Vec::new(),
+            names: Vec::with_capacity(source_len / 64),
             names_map: FxHashMap::default(),
-            tokens: Vec::new(),
+            tokens: Vec::with_capacity(source_len / 4),
             last_generated_update: 0,
             last_position: None,
             line_offset_tables,
@@ -381,7 +383,7 @@ impl<'a> SourcemapBuilder<'a> {
     }
 
     fn generate_line_offset_tables(content: &str) -> LineOffsetTables {
-        let mut lines = vec![];
+        let mut lines = Vec::with_capacity(content.len() / 24 + 1);
         let mut column_offsets = IndexVec::new();
 
         // Used as a buffer to reduce memory reallocations.