Fix loader include under V3

[antono3]

Summary

• resolve the private Volk loader bridge through @VMODROOT so generated C does not depend on the compiler's working directory
• add a pinned, authoritative V3 compile-and-test matrix entry using -new-compiler
• retain the live V master compatibility lane as an advisory drift check

Why

v_vulkan_video exposed this under V3: generated src.c included loader_bridge.h without an include path and GCC could not find it. The released compiler happened to preserve the module-relative lookup. Using V's documented module-root path makes the binding portable across both frontends.

Local verification

• V 0.5.2 generator compile smoke with GCC
• V 0.5.2 ergonomic tests: 4/4 passed
• workflow YAML parse and git diff --check

The PR matrix provides the authoritative pinned V3 regression check.