docs: track foreign declaration frontend slice

[vycdev2]

Summary

• link the C FFI contract's first parser/formatter/resolver/checker slice to its implementation tracker
• distinguish the completed design contract and frontend prerequisite from still-pending jett_bind generator/CLI work
• align the design, architecture, and progress references with that staged status

Verification

• cargo fmt --check — passed
• cargo build — passed
• cargo test -q — passed (workspace suite, including 543 driver fixture tests)
• git diff --check — passed

Risk

• low: documentation-only tracking changes; no language or compiler behavior changes

Closes #173

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.