Fix concurrent initialization directory creation

[VolkerChristian]

Architectural invariant

Concurrent processes sharing the configuration, state and runtime directories must initialize successfully when another process creates a directory first. Only real filesystem errors are failures; ownership and permissions are changed only by the creator.

Fix

Remove the racy exists prechecks and the false-result error branches from Config::init. std::filesystem::create_directories is now the sole authority: true means newly created, false means an existing directory, and actual filesystem errors still propagate. This also rejects an existing non-directory instead of silently accepting it.

No retries, test serialization, precreated directories, workflow workarounds, new production helpers or public API/ABI changes.

Regression and verification

• Eight boundary scenarios exercise fresh/existing directories, a competing creator for each of the three paths, and an obstructing file for each path.
• Uses the existing test-only libc interposition pattern to force the exact losing mkdir result deterministically, without production hooks. Verifies existing and competing-creator permissions are preserved.
• Before the fix: fresh/existing passed, all three forced races and all three file obstructions failed.
• After the fix: 220/220 CTests passed, unprivileged, GCC 15.3.0 on Debian trixie, parallel 8, fresh HOME; includes installed-consumer verification.
• clang-format 19, repository cmake-format configuration, and git diff --check passed.
• Production changes: +3/-12 (net -9 lines). Test changes: +172/-0 (includes existing-style copyright headers and CMake registration).

This resolves the startup race observed in default-branch Verify run https://github.com/SNodeC/snode.c/actions/runs/38068740872 before the development SDK could be published. Package workflows, licence files, READMEs, VERSION and stable release tags are unchanged. CodexUI/AutoTalk test fixes remain explicitly deferred.