[wieslawsoltes]

Problem

The exact Windows runtime build stops on three portability defects: an oversized stylesheet mutation JavaScript literal, an unconditional POSIX resource header in the CSS URL oracle, and a 263-character generated object path for the Linux external-image ABI contract.

Change

• Partition the 23.7 KiB stylesheet mutation setup into two bounded runtime-joined literals. The resulting JavaScript source is byte-for-byte unchanged.
• Limit <sys/resource.h> and getrusage to Apple/Linux. Unsupported platforms report zero, preserving the optional RSS gate.
• Shorten only the CMake build target for the external-image contract, reducing its estimated Windows object path to 245 characters while preserving the public CTest name and labels.

Validation

• All JavaScript raw literals in the browser DOM test are below 12 KiB.
• The renamed target builds locally and remains registered as webscene_graphics_linux_external_image_contract_tests.
• Diff and CMake generation checks pass; exact Windows compilation is the stack gate.

Closes #846.