feat(kernel): R7b R walker — rlang module, tree-sitter-r 1.2.0 crate pin, r default-routed (#1383)

R7b batch 4 #1 (docs/design/r-kernel-port-checklist.md is the authoritative
quirk list; survey + probe record therein). The lightest-shared-surface,
heaviest-hook port: languages/r.ts works entirely through the visitNode hook
(every type list empty except callTypes:['call']), so the walker is a file
node + a faithful hook transcription + the generic extractCall + pre-order
recursion — four shared machineries (value-refs, static-member reads, type
annotations, fn-ref capture) are dead by language gates and stay dead.

Grammar prep is the first true no-op of the arc: the crates.io tree-sitter-r
1.2.0 tarball ships parser.c AND scanner.c sha-identical to the r-lib v1.2.0
tag the vendored wasm was built from — crate pin only, no wasm change, no
bump gate; kernel-grammar-parity gains the r row (ABI 14, same-revision).

Preserved bug-for-bug (all probe-pinned): calls "return" on every return(x)
(named node in v1.2.0), the import quintet's silent dynamic-arg consumption
vs class/generic fall-through asymmetry, library(help = pkg) importing the
named arg, class-idiom variable suppression by callee name, chained/right-
assign/precedence-ghost gaps, env$fn body-leak-to-file, raw-text callees
verbatim (pkg::fn, obj$meth, "strfn" quotes kept, (handler) conversion),
duplicate same-(kind,name,line) ids, roxygen dropped entirely, UTF-16
columns/slices.

Gates: parity sweeps first-run 0-diff on AnomalyDetection/dplyr/ggplot2/
shiny (838 files; deferrals exactly 0/0/0/1 — the 1 is the moustache-
template pseudo-R file, both-arm) — kernel-parity.mjs gained lowercased-
extension matching so .R files sweep (matches detectLanguage routing);
full-init dumps byte-identical kernel-vs-wasm on dplyr/ggplot2/shiny;
kernel-r-parity suite (torture fixture + in-memory CRLF + BOM variants +
defer pin + kernel-arm quirk pins); full suite 2,638 green ×2 with
CODEGRAPH_KERNEL_EXPECT=1. DEFAULT_ROUTED += r (16 langs).

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

scripts/kernel-parity.mjs

@@ -48,7 +48,7 @@ if (paths.length === 0) {
   process.exit(2);
 }
 
-const KERNEL_LANGS = new Set(['typescript', 'tsx', 'javascript', 'jsx', 'java', 'python', 'go', 'c', 'cpp', 'rust', 'csharp', 'ruby', 'php', 'swift', 'kotlin']);
+const KERNEL_LANGS = new Set(['typescript', 'tsx', 'javascript', 'jsx', 'java', 'python', 'go', 'c', 'cpp', 'rust', 'csharp', 'ruby', 'php', 'swift', 'kotlin', 'r']);
 const EXTS = new Map([
   ['.ts', 'typescript'], ['.mts', 'typescript'], ['.cts', 'typescript'],
   ['.tsx', 'tsx'], ['.js', 'javascript'], ['.mjs', 'javascript'],
@@ -64,6 +64,7 @@ const EXTS = new Map([
   ['.php', 'php'], ['.module', 'php'], ['.install', 'php'], ['.theme', 'php'], ['.inc', 'php'], // R7b
   ['.swift', 'swift'], // R7b
   ['.kt', 'kotlin'], ['.kts', 'kotlin'], // R7b
+  ['.r', 'r'], // R7b batch 4 — real-world casing is `.R`; extname lowercased below
 ]);
 
 /** Collect candidate files. */
@@ -78,8 +79,10 @@ function collect(p, out) {
     const base = path.basename(p);
     if (base === 'node_modules' || base === '.git' || base === 'dist' || base === '.codegraph') return;
     for (const e of fs.readdirSync(p)) collect(path.join(p, e), out);
-  } else if (EXTS.has(path.extname(p))) {
-    const lang = EXTS.get(path.extname(p));
+  } else if (EXTS.has(path.extname(p).toLowerCase())) {
+    // Lowercased to match the real indexer's routing (detectLanguage
+    // lowercases the extension — `.R` is the dominant real-world casing).
+    const lang = EXTS.get(path.extname(p).toLowerCase());
     // 'detect' (.h) resolves per file in the run loop; under --lang it rides
     // along whenever either C-family language is requested.
     const passes =