feat(extract): add regex-based R (.r/.R) extractor (#1689)

[Sanjays2402]

Summary

Adds a real extractor for R (.r / .R), resolving the actionable half of #1689: R files were classified as code (.r is in CODE_EXTENSIONS) but had no extractor wired into _DISPATCH, so every R file silently contributed zero nodes to the graph. The #1689 warning (already at HEAD) surfaced the gap; this PR closes it.

Why regex, not tree-sitter

Unlike every other language graphify depends on, there is no standalone tree-sitter-r distribution on PyPI (tree-sitter-r / tree-sitter-rlang both 404; verified via the PyPI JSON API and pip index). The only pip-installable R grammar is the ~165-grammar tree-sitter-language-pack, which cuts hard against this repo's one-dedicated-wheel-per-language convention.

So this follows the existing regex-fallback precedent already used for Pascal (#781) and the non-tree-sitter path for DM (#1104): a self-contained, zero-dependency regex extractor in graphify/extractors/r.py. pyproject.toml is untouched — no new dependency.

An AST-quality upgrade is a clean future follow-up if/when a standalone R grammar wheel becomes available; the module is structured so that swap is localized.

What it extracts

Construct	Example	Emitted
Functions (all binding forms)	f <- function(x), g = function(x), h <<- function(), k <- \(x) x	contains node/edge
Package imports	library(dplyr), require(ggplot2), requireNamespace(...), loadNamespace(...)	imports (context=import)
File includes	source("helpers.R")	imports (context=import)
S4 / reference classes	setClass("Circle", ...), setRefClass(...)	contains node
S4 inheritance	setClass("Circle", contains = "Shape")	inherits edge
S4 generics	setGeneric("area", ...)	contains node
S4 methods	setMethod("area", "Circle", ...)	method edge (class → method)
Intra-file calls	local fn → local fn	calls (context=call)

The extractor is string- and comment-aware (a # inside a string literal is not a comment; braces/parens inside strings don't affect body-span matching), and skips accessor/namespace-qualified call targets (obj$m(), obj@m(), pkg::fn()) so only genuine local free-function calls become calls edges.

Wiring

• graphify/extractors/r.py — new module (mirrors extractors/elixir.py layout, reuses base.py helpers _make_id / _file_stem).
• extract.py — import + re-export extract_r; .r/.R → _DISPATCH; language-name map entry; Rscript → _SHEBANG_DISPATCH (so #!/usr/bin/env Rscript extensionless scripts route correctly); updated the #1689 warning comment (it no longer applies to .r).

Tests

• tests/fixtures/sample.R — new fixture exercising every construct.
• tests/test_languages.py — 12 new R tests (functions, \(x) lambda, imports incl. source(), S4 classes, inheritance, generics/methods, calls, call-context, no-dangling-edges).
• tests/test_extract.py — updated the two tests that used .r/.R as their "code file with no AST extractor" example (that's no longer true) to use .ets (ArkTS), which remains genuinely unsupported.

Local run (on v8): tests/test_languages.py + tests/test_extract.py = 442 passed, 14 skipped, no regressions. The 12 new R tests and the two updated #1689 tests all pass.

Closes the extractor half of #1689.