Add deterministic R extraction

[forhappy]

Summary

• add deterministic structural extraction for .r and .R source files
• recognize extensionless Rscript executables
• extract package imports, named and nested functions, local calls, and unresolved raw calls
• preserve warnings and progress coverage using genuinely unsupported .ejs fixtures

Why

Graphify classifies R files and Rscript executables as code, but previously had no extractor for them. Those files therefore contributed no nodes and appeared in the unsupported-language warning. Compass already implements the same deterministic extraction contract, so Graphify needs the matching implementation for extraction parity.

User impact

R repositories now contribute file/function nodes, package-import edges, nested-function containment, and local call relationships without requiring an optional tree-sitter grammar.

Validation

• 344 passed across tests/test_extract.py, tests/test_extractors_registry.py, and tests/test_detect.py
• 3 passed in the dedicated R extractor suite
• Ruff checks passed for all changed Python files
• python3 -m compileall -q graphify/extractors/r.py
• git diff --check
• graphify update .