feat(extraction): R language support (#828) (#839)

R has no declaration syntax — everything is an expression — so the
extractor works through the visitNode hook: functions in every
assignment form (incl. nested, attributed to their enclosing scope),
top-level variables/constants, library()/require() imports and
source() file references (claimed, Lua-style), S4/RefClass/R6/ggproto
classes with their methods and extends edges, setGeneric/setMethod.
Grammar vendored from r-lib/tree-sitter-r v1.2.0 (ABI 14; npm package
is a security placeholder, tree-sitter-wasms has no R).

Benchmarked on AnomalyDetection (8/8 named defs), dplyr (1027 fns),
ggplot2 (150 ggproto classes / 597 methods / 128 extends edges —
adding ggproto mid-bench flipped the large-repo A/B from a regression
to 2.4x faster than the no-codegraph arm).

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

src/extraction/languages/index.ts

@@ -24,6 +24,7 @@ import { dartExtractor } from './dart';
 import { pascalExtractor } from './pascal';
 import { scalaExtractor } from './scala';
 import { luaExtractor } from './lua';
+import { rExtractor } from './r';
 import { luauExtractor } from './luau';
 import { objcExtractor } from './objc';
 
@@ -47,6 +48,7 @@ export const EXTRACTORS: Partial<Record<Language, LanguageExtractor>> = {
   pascal: pascalExtractor,
   scala: scalaExtractor,
   lua: luaExtractor,
+  r: rExtractor,
   luau: luauExtractor,
   objc: objcExtractor,
 };