fix(rust): index unit structs — bodiless is a definition, not a forward decl (#1800)

Land upstream PR #1514 for issue #1513 by cherry-picking
ctype_lab's a94c9dc94eee348bf63c7e2dd567c0b43577678a.

Keep allowBodilessStruct as a Rust-only opt-in, with matching behavior in
the wasm/TypeScript walker and native kernel. Create the node before
checking for a body and walk members only when present. Resolve against
main's shared struct/union walker while retaining its stack guard, kinds,
fields, and existing impl-receiver fixes.

Verified FAIL to PASS on Linux x64 with Node 22.19.0 after rebuilding via
tsc, copy-assets, and build:kernel. The identical fixture on main 7b339373
had two structs and two implements edges; fresh wasm (CODEGRAPH_KERNEL=0)
and native indexes now have three of each. UnitStruct and its Greet
implements edge are recovered; tuple and brace structs remain intact.
The native run loaded the rebuilt kernel and completed without fallback.

Focused extraction.test.ts and kernel-rustlang-parity.test.ts runs:
645 tests passed with the kernel disabled, and 645 with it enabled;
all three parity tests ran in each configuration, with no skips.

(cherry picked from commit a94c9dc94eee348bf63c7e2dd567c0b43577678a)

Co-authored-by: ctype_lab <cksgud1226@gmail.com>
Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

__tests__/kernel-rustlang-parity.test.ts

@@ -5,7 +5,8 @@
  * SAME ExtractionResult as the wasm TreeSitterExtractor — nodes, edges, and
  * unresolved refs compared as canonicalized multisets — over the checked-in
  * torture fixture (torture.rs: impl/trait quirks incl. generic / lifetime /
- * reference / scoped / generic-trait impl receivers (#1588), unit-struct skip, phantom
+ * reference / scoped / generic-trait impl receivers (#1588), unit structs
+ * (a bodiless struct IS a definition — both walkers mint a node), phantom
  * const identifiers, use-binding refs incl. nested groups + wildcard-emits-
  * nothing, chained-call re-encode, turbofish, Rocket route macros body-only,
  * fn-ref shapes, value-ref shadowing, attribute-broken docstrings, dead-code