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>

codegraph-kernel/src/lib.rs

@@ -27,6 +27,7 @@ mod java;
 mod kotlin;
 mod langs;
 mod php;
+mod rlang;
 mod ruby;
 mod rustlang;
 mod swift;
@@ -223,6 +224,7 @@ pub fn extract_file(file_path: String, content: String, language: String) -> Res
         "php" => php::extract(&file_path, &content).map_err(Error::from_reason)?,
         "swift" => swift::extract(&file_path, &content).map_err(Error::from_reason)?,
         "kotlin" => kotlin::extract(&file_path, &content).map_err(Error::from_reason)?,
+        "r" => rlang::extract(&file_path, &content).map_err(Error::from_reason)?,
         _ => tsjs::extract(&file_path, &content, &language).map_err(Error::from_reason)?,
     };
     Ok(ExtractBuffers {