feat(kernel): R7b C# walker — csharp module, tree-sitter-c-sharp 0.23.5 pin, csharp default-routed (#1378)

Second R7b port, checklist-first recipe (docs/design/csharp-kernel-port-checklist.md;
parity passed FIRST RUN again). No grammar bump — the #717 vendored wasm verified
table-identical to crate 0.23.5 (ABI 15, STATE_COUNT 8053, node-kind + field tables);
first port with no grammar-prep step. The #237 #if-blanking preParse stays TS-side
via the existing route-point hoist.

Walker preserves bug-for-bug: the single-namespace-node quirks (second namespace
nests under the first, nested namespaces leave no trace, import refs hang off the
namespace node), raw member-access callee texts (this./base./literal receivers,
multi-line fluent chains) with unconditional chain re-encode, deliberate emission
holes (property accessor/expression bodies, ctor initializers, delegates/events/
operators/indexers/local functions, top-level locals), garbage extends refs
((repo) primary-ctor args, BaseDto(Name) record bases, enum : byte), the alias-
import moduleName quirks, nameof-as-call, CSHARP fn-ref spec (+= subscription,
this.X bare-name form, argument layer, initializer lists), C# type-ref engine
(nested-generic returnType failure included), and value-ref shadow pruning.

Gates: sweeps 0-diff serilog 211/216 / Newtonsoft.Json 914/945 / jellyfin
2104/2105 (deferrals match the survey's per-repo predictions — both-arm #if
damage; default --max-deferral 0.1 holds, no c/cpp exemption); full-init dumps
byte-identical ×3 (14.0k/109.1k/210.8k lines); kernel-csharp-parity suite
(torture ×3 + CRLF variants + 8 micro-pins + defer) + csharp grammar-parity row;
full suite 2,608 ×2 under CODEGRAPH_KERNEL_EXPECT=1. DEFAULT_ROUTED += csharp
(11 langs).

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

src/extraction/kernel/index.ts

@@ -54,6 +54,11 @@ const DEFAULT_ROUTED: ReadonlySet<Language> = new Set<Language>([
   // parser crates, 18%) error on BOTH arms — grammar-inherent, not a walker
   // signal.
   'rust',
+  // R7b (2026-07-20): parity swept 0-diff on serilog/Newtonsoft.Json/jellyfin
+  // (3,229 files byte-parity) + full-init dump-diffs byte-identical ×3.
+  // Deferral 0.05–3.3% — both-branches-kept `#if` damage that errors on BOTH
+  // arms (the preParse blanking hoist keeps the kernel's input identical).
+  'csharp',
 ]);
 
 /**