feat(adapters): parse Anthropic prompts

[kev1n77]

Summary

• add an Anthropic adapter for Messages and legacy Complete requests
• map system instructions, user/assistant/tool messages, text, image/document references, tool use/results, tool schemas, and generation options into Prompt IR
• preserve unsupported content as Unknown with structured partial issues and degrade truncated bodies to Raw
• copy Anthropic version/beta headers and request controls into vendor evidence without duplicating inline base64 data
• register the adapter by default and move the unknown-provider regression fixture to Gemini
• update engineering progress to 30% and PAR-004 request-side completion to 70%

Verification

• cargo fmt --all -- --check
• cargo test --workspace --all-targets
• cargo clippy --workspace --all-targets -- -D warnings
• cargo fmt --manifest-path apps/desktop/src-tauri/Cargo.toml -- --check
• cargo test --manifest-path apps/desktop/src-tauri/Cargo.toml
• cargo clippy --manifest-path apps/desktop/src-tauri/Cargo.toml --all-targets -- -D warnings
• npm run desktop:check
• npm run desktop:build
• python -m unittest discover -s sidecars/mitmproxy/tests -v

Scope note

Anthropic response SSE reconstruction remains pending because the current encrypted storage contract persists request envelopes only. This PR completes the request-side adapter and records the dependency explicitly in the progress tracker.