perf(swc_ts_fast_strip): reduce repeated binary-search token lookups in TsStrip

[kdy1]

Summary

TsStrip performs many token-position lookups via binary search, which likely adds avoidable log-factor overhead.

Evidence

• Core lookup helpers use binary search every time:
  • crates/swc_ts_fast_strip/src/lib.rs:627-633 (get_next_token_index)
  • crates/swc_ts_fast_strip/src/lib.rs:639-644 (get_prev_token_index)
• These lookups are called from many visitor paths (visit_arrow_expr, visit_class_prop, visit_params, visit_getter_prop, etc.).

Why this is a performance risk

TsStrip does a lot of local token queries while walking the AST. Repeated binary_search_by_key calls can add up to O(k log n) token-lookup cost (k = number of strip/fix operations).

Suggested direction

• Introduce an index cursor in TsStrip and advance it as traversal moves forward in source order (amortized near O(1) local lookups).
• Alternatively, precompute a compact position->token-index table for the span boundaries used by stripping logic.

Benchmark idea

Use the existing typescript/fast-strip benchmark and compare time spent in token lookup helpers before/after cursorization.