Restore macOS 12 NuGet package CI for floating form serialization #908

Reproduction

WebScene main has repeatedly failed the NuGet packages / Build osx-arm64 job (for example runs 35585510953, 35581500980, and 35574320848). Focused PR #907 reproduced the same failure in run 35692037242, while its related CI, contracts, and portable V8 checks passed. The package lane compiles for macOS 12.0, but experiments/WebScene.NativeEngine.Probe/native/webscene_native_form_state.h uses floating-point std::from_chars at line 117 (annotated available only from macOS 26.0) and floating-point std::to_chars at lines 358 and 430 (available only from macOS 13.3). The compiler stops before package tests or artifacts are produced. This predates #907 and is not a CSS regression.

Proposed fix and acceptance

Keep the macOS 12 deployment target. Provide a locale-independent, finite-only floating parse/format implementation for targets where libc++ floating charconv is unavailable; preserve numeric and time-input serialization semantics, shortest round-trip output where required, and integer charconv paths. Add focused number/range/time edge and locale tests, a compile/link gate at macOS 12, and a bounded performance comparison. Then rerun the NuGet osx-arm64 lane plus related native form and Code OSS acceptance checks. Do not raise the deployment target merely to make CI green.