Harden R language extraction parity

[forhappy]

What

• add a deterministic R extractor with Graphify-compatible entities and relationships
• register .r and extensionless Rscript sources
• add bidirectional dispatch-surface coverage and exact R extraction parity tests
• cover invalid UTF-8 input and Rscript shebang dispatch

Why

Compass lacked R extraction even though Graphify classified R as code, leaving the two implementations out of sync.

Impact

Compass can extract R without an R runtime or parser dependency, and the parity suite now guards against future one-sided language registration.

Companion PR: Graphify-Labs/graphify#2146

Checks

• all Compass language implementation and registry tests passed
• cargo check -p compass-parity passed
• Compass parity suite: 98/98 passed in the integrated checkout
• cargo fmt --check and git diff --check passed