Add Rust-owned C baseline extraction

[jununfly]

Summary

• Add Rust-owned C baseline extraction for functions, structs, enums, enum members, typedefs, includes, calls, and variables.
• Mark C as a rust-hybrid Rust-owned language and classify ambiguous .h files by content before assigning ownership.
• Remove the migrated TypeScript-owned C extractor path while keeping the C++ TypeScript extractor.
• Record cJSON real-corpus validation and update the Rust-owned migration roadmap.
• Retain the ZAgenticLoop scaffold and closeout state per Human Gate decision.

Validation

• cargo build --package zcodegraph-core
• npm run build
• npx vitest run __tests__/rust-index-engine-cli-language-smoke.test.ts __tests__/rust-index-engine-cli-fallback.test.ts __tests__/rust-index-engine-cli-engine.test.ts
• npx vitest run __tests__/extraction.test.ts -t "C/C\\+\\+ imports|Objective-C|detect language"
• git diff --check
• Real corpus: DaveGamble/cJSON at fb16e5c, 121 indexed files, 3,581 nodes, 7,000 edges, engineByLanguage.c = rust, no Rust-owned C gap diagnostics.

Post-Merge

• Delete branch zjal/rust-owned-c-baseline after merge.