Restore C11 labels and C declaration ownership

[Alexbeav]

GCC 9 rejects the local declaration immediately after retry_candidates: in overlay_loader.c. Add the required null statement. It then reaches linking and exposes two local C++ declarations of fntrace_is_game_started; remove those duplicate declarations so the existing fntrace.h C declaration owns the linkage.

Two source-owned checks cover the actual label/declaration excerpt under -std=c11 -pedantic-errors and the header-owned declaration. They fail on the unchanged source and pass on this branch. They are registered in CTest.

Native Linux GCC 9 full-runtime controls:

• Unchanged base and corrected candidate: the base fails at the C label, while this exact candidate compiles and links all 484 actions and passes both focused tests.
• Label-only control: fixing only the label reaches the final link and fails on the C++-mangled fntrace_is_game_started(), independently demonstrating the second correction.

The CI workflow is on a separate build-only branch and is not part of this contribution.

Validation and limits

• Exact base: 8d56cf659fd84367c3f778e17851674ac7896371; review head: d8725466bcbae7b235f33a6d334bfd455e39e7f6.
• Native Windows GCC 16 runtime builds and passes 70/70 enabled runtime tests; two additional tests are disabled in the source configuration. The build has UI, setup wizard, debug tools, netplay, rewind and Vulkan disabled.
• Owned Ghost in the Shell (SCES-01050) with owned SCPH1001: verification, current-source generation, native build and an isolated 50-second startup pass; normal SDL window close at frame 2456, process exit 0. Executable SHA-256 ebf755b918c52d9625e08aedb31e3b6d4baaeebbbdfe6022d5a573257e46fe67. This is bounded startup and normal shutdown, not gameplay completion or listening acceptance. Audio used SDL dummy output. No native Linux/macOS game route is claimed.
• The unchanged current recompiler builds and has 58/61 enabled tests passing, with three pre-existing baseline failures (dirty_text_continuation_guards, aot_overlay_discovery, release_zip) and three disabled tests. This branch does not change that recompiler source. Those baseline failures are not reported as passes.
• Diff audit: general source repair, synthetic tests and documentation only; no retail disc/BIOS, generated game code, player data or private tooling.

Scope and fork review

Fork review #26 inspected this exact head against immutable review-base/mstan-8d56cf659fd8. Cubic reported no issues on the exact head (review IDs: 5132944873); no unresolved findings remain. The public PR branch points to the same commit. Upstream master was fetched again and remains the recorded base. This contribution contains 1 commit and 6 changed files.

• docs/C11_SOURCE_COMPATIBILITY.md: behavior and verification documentation.
• runtime/CMakeLists.txt: source-owned regression and test registration.
• runtime/src/main.cpp: bounded correction.
• runtime/src/overlay_loader.c: bounded correction.
• runtime/tests/test_fntrace_c_linkage.py: source-owned regression and test registration.
• runtime/tests/test_overlay_retry_c11.py: source-owned regression and test registration.

Developed with AI assistance; validated as described (test evidence in PR body). AI writes the code and the PR, but I always test before I send something up. Happy to iterate on this process with your feedback.