perf(concat): hoist per-token chunk state out of the per-iteration closure

[Boshen]

Summary

`add_sourcemap`'s per-token loop captured `&mut self` inside the `get_source_id().map(...)` / `get_name_id().map(...)` closures so it could update `token_chunk_prev_source_id` / `token_chunk_prev_name_id` on every iteration. That forced the compiler to round-trip the `prev_*` fields through memory (load, mutate, store via the `self` pointer) for every token rather than keeping them in registers.

Track the running prev values in plain locals, then write them back to `self` once at the end. Also handle the first token separately so the `i == 0` dedup check no longer runs every iteration.

Behavior, including the duplicate-token de-duplication at the concatenation seam, is unchanged.

[codspeed-hq]

Merging this PR will improve performance by 2.5%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 4 improved benchmarks
✅ 12 untouched benchmarks
⏩ 5 skipped benchmarks¹

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	add_sourcemap_loop	363 µs	353 µs	+2.83%
⚡	from_sourcemaps	344.3 µs	334.3 µs	+3.02%
⚡	lookup_table[real_medium]	1.5 µs	1.5 µs	+1.97%
⚡	lookup_table[real_small]	1.4 µs	1.3 µs	+2.18%

Tip

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_{Comparing perf/concat-hoist-state (15efed1) with main (9323530)}

Footnotes

1. 5 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩