add tests for core/list#2
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Three related allocation removals along the JSON number lex chain, all stemming from the same observation: the native target boxes every returned struct / tuple / enum payload by default. ## 1. Drop the `(Double, StringView?)` tuple The number-lex chain (`lex_zero` / `lex_decimal_*` / `lex_number_end` / `lex_integer_end`) returned `(Double, StringView?)`, where the second component is the source-text view that's only ever `Some(_)` on the infinity-overflow path. Native boxes every tuple, so every parsed JSON number cost one heap allocation just to carry a value that was structurally `None`. Side-channel the optional view through a new private `ParseContext.last_number_repr` field. Each leaf return writes it (usually `None`); `lex_main.mbt` reads + clears when constructing the `Number` token. ## 2. Stack-allocate `JsonNumberScan` via `#valtype` `scan_json_number` builds + returns a five-field `JsonNumberScan` struct on every JSON number, used immediately by `lex_number_end` and discarded. Without `#valtype` the native target boxes that struct as a ~32-byte heap object per call. ## 3. NaN sentinel for `try_fast_double` `JsonNumberScan::try_fast_double` returned `Double?` — `Some(d)` on the fast path, `None` to fall back to strconv. The fast path can only produce 0 or a finite `Double` (the `checked_mul` guard rules out infinity), so `NaN` is a free sentinel. Returning a plain `Double` (with NaN meaning "not handled") skips the boxed `Option<Double>` allocation on every JSON number that hits this path. ## Numbers Measured on a native-target alloc profiler over mizchi/pprof-mbt's bench suite (`--sample-rate 100`): | bench | metric | before | after | Δ | |---|---|---|---|---| | `json_numbers` (10 k integers × 30) | allocs | 1 200 200 | 600 200 | **−50 %** | | | bytes | 18.31 MB | 9.16 MB | **−50 %** | | `json_parse` (1 000-obj × 50) | allocs | 3 250 200 | 2 300 200 | **−29 %** | | | bytes | 58.56 MB | 46.93 MB | **−20 %** | (`moonbitlang#3` only fires for non-integer / non-overflow numbers, so it adds 100 k allocs / 681 kB on top of #1+moonbitlang#2 on `json_parse` and nothing on `json_numbers`.) Tests: `moon test --target native -p json` and `moon test --target wasm-gc -p json` both pass (171 / 171 each). `moon fmt --check` clean.
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