ci: Add a compile the public headers job (#1387)

[kallal79]

This PR ensures that all public C headers in secp256k1 are strictly self-contained and independently compilable without requiring downstream projects to manually include missing dependencies (like <stddef.h> or <stdint.h>).

Changes included:

1. GitHub Actions (ci.yml): Added a dedicated c_headers_debian job that runs gcc and clang in -std=c89 and -pedantic-errors modes to precompile every public header, mirroring the existing C++ checks. Also updated the Windows MSVC job to test headers using -TC (C-mode).
2. Offline Tests (test_public_headers.c): Created a dedicated C test file that explicitly #includes every public header conditionally based on enabled modules (e.g., ENABLE_MODULE_ECDH). This test is fully integrated into both CMakeLists.txt and Makefile.am so it will automatically be verified during local make check and ctest runs.

[real-or-random]

• CI fails here, see for example https://github.com/bitcoin-core/secp256k1/actions/runs/27440362791/job/81112805983?pr=1870#step:3:1600
• I don't see what the purpose of the src/test_public_headers.c (and its CMake integration) is. Is the goal to make sure that the headers compile even when they're all included in a single compilation unit?