Make retained inset parsing portable on macOS

[wieslawsoltes]

Problem

The retained inset clip parser uses floating-point std::from_chars. Apple libc++ only exposes that overload on newer macOS deployment targets, so the current WebScene package fails to compile for the supported macOS target.

Change

• add a locale-independent CSS ASCII-number prefix parser for retained inset lengths
• preserve the accepted px, %, unitless-zero, sign, decimal, and exponent forms
• reject partial, non-finite, overflowed, unsupported-unit, trailing-whitespace, and embedded-NUL values
• use the portable parser in retained inset clip resolution
• build the focused parser contract in every native configuration

Validation

• AppleClang build with CMAKE_OSX_DEPLOYMENT_TARGET=12.0 and V8 disabled
• full V8-free CTest: 21/21 passed
• focused CSS number and HTML parser contracts passed
• 100,000 parses: about 1.10 ms against a 500 ms budget
• git diff --check

This PR addresses the macOS package compile failure only. Linux CSS timing failures remain tracked in #238, and the independent Windows IndexedDB literal failure remains tracked in #301.

Closes #302