#1652 Fix Linux CUDA builds with selectable GCC (#1645)

本文

Fixes #1645.

Adds --gcc gcc-14 (or STRATA_GCC) to select matching C, C++ and CUDA host compilers for the engine and GPU image encoder. Compiler changes invalidate reuse and use separate CMake build directories.

Validation on Ubuntu 24.04.5 / RTX 3070:
- Reproduced CUDA 12.9 rejecting real GCC 15.
- Actual setup.sh with GCC 14 built both binaries and completed setup.
- GCC 14 -> 13 -> 14 transitions passed; original binary hashes restored.
- Text inference passed. Default vision failed with little VRAM remaining; prefill and image tokens set to 128 passed.
- Linux: 431 unit tests passed, 1 Windows-only skip. Windows: 432 unit tests passed (no Windows CUDA build).
- Repaired three existing Linux test-isolation failures.

Full reproduction and limitations: docs/SETUP_GCC_VALIDATION.txt.