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perf(json): drop the (Double, StringView?) tuple via a #valtype struct

The number-lexing chain returned a `(Double, StringView?)` tuple, which is
heap-boxed once per parsed number. Replace it with a `#valtype` struct
`LexedNumber { repr, value }` that stays stack-allocated on the native target,
so the common integer/float path no longer boxes a result per number -- while
keeping a plain by-value return (no signature-flow changes, no mutable side
channel on `ParseContext`).

Bench (native, n=10000 array):
  int:   ~320 us -> ~306 us  (~4-5%)
  float: ~437 us -> ~424 us  (~3%)

Field order matters: `repr` (the reference field) must precede `value` (the
Double), otherwise the Wasm backend currently mis-compiles the destructure in
`lex_value` (`i32.wrap_i64 expected i64, found f64`).

Adds json/number_bench_test.mbt.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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