0.0.12 joint release: maintained clangd correctness, performance and distribution #45

Implement the approved joint 0.0.12 release plan in .agents/docs/2026-10-07-0.0.12-joint-release-plan.md.

The product and maintained clangd must progress through G0 evidence/gates, semantic correctness and stability, measured optimization and formatting/extensions, four-platform distribution, and candidate review. Linux is validated first, then PR CI supplies other platform evidence. Draft PRs remain unmerged and no release is published. Experimental patch labels, empty crash corpora, or completion answers without required symbols are insufficient release evidence.