Make retained inset length parsing portable below macOS 26

[wieslawsoltes]

Parents: #256, CSS epic #235

Reproduction

The required macOS arm64 runtime-package build fails on current main 70a2db4c in run 35254142552, job 105313709946:

webscene_native_dom_scene.inc:443:34: error: 'from_chars' is unavailable: introduced in macOS 26.0

The call parses floating-point clip-path: inset(...) lengths. Integer std::from_chars is portable here; libc++ floating-point from_chars is unavailable at the package deployment target. The implementation predates PR #300 and reproduces in its post-merge package run.

Required fix

• Replace the unavailable floating parser with a locale-independent, bounded parser that accepts the same CSS numeric grammar already admitted by the inset path.
• Preserve complete-token validation, finite-only values, optional leading +, zero-unit handling, px, and % behavior.
• Reject whitespace tails, partial numbers, NaN/Inf, overflow, unsupported units, and embedded NUL without reading beyond the input view.
• Reuse a shared portable CSS-number helper if one already meets those rules; do not add locale-sensitive parsing to scene publication.

Acceptance

• Focused positive/negative parser and retained inset-paint tests pass.
• macOS package target compiles at the declared deployment target.
• Linux/Windows portability builds pass the same parser contract.
• Native/Chromium inset geometry/pixels and the existing 4,096-effect lifecycle/performance gate remain unchanged.

[wieslawsoltes]

Focused implementation is open as PR #306 at rebased head 9131e5c9 on current main 50443f5b.

It replaces deployment-target-dependent floating std::from_chars with a locale-independent CSS ASCII-number prefix parser while preserving the accepted retained-inset forms and rejecting partial, non-finite, overflowed, unsupported-unit, trailing-whitespace, and embedded-NUL inputs.

The macOS-12 AppleClang build, focused parser contract, full pre-rebase V8-free 21/21 suite, and 100,000-parse performance gate pass locally. Only the directly related macOS/package hosted lanes remain in the queue; superseded rebase runs were cancelled.