rust: unit structs (`struct Unit;`) are not indexed, taking their `impl Trait for` edges with them

[ctype-lab]

codegraph extracts no symbol at all for a Rust unit struct (struct Unit;),
so the type is absent from the graph — and so is everything attached to it. The
most visible loss is impl SomeTrait for Unit: the edge never forms, because its
source endpoint does not exist.

Brace structs (struct S { .. }) and tuple structs (struct S(u32);) are
extracted normally. Only the unit form is affected.

The grammar parses the file correctly, so this is in the symbol-extraction layer,
not the grammar.

Environment

• codegraph: 1.5.0 (npm), reproduced again on main @ d6d1728
• Backend: node:sqlite (WAL)
• Platform: Linux x64
• Both extraction paths affected: the native kernel and the wasm/TS walker
  (CODEGRAPH_KERNEL=0)

Repro

src/lib.rs:

pub struct UnitStruct;
pub struct TupleStruct(pub u32);
pub struct BraceStruct { pub x: u32 }

pub trait Greet { fn hi(&self) -> String; }

impl Greet for UnitStruct  { fn hi(&self) -> String { "unit".into()  } }
impl Greet for TupleStruct { fn hi(&self) -> String { "tuple".into() } }
impl Greet for BraceStruct { fn hi(&self) -> String { "brace".into() } }

codegraph init .
sqlite3 .codegraph/codegraph.db \
  "SELECT kind, name FROM nodes WHERE kind IN ('struct','trait') ORDER BY name;"

Expected

Three struct nodes and three implements edges.

Actual

Two of each — UnitStruct is missing, and with it the Greet <- UnitStruct
realization.

struct  BraceStruct
struct  TupleStruct
trait   Greet

Greet <|.. TupleStruct
Greet <|.. BraceStruct

Impact

Unit structs are the idiom for zero-sized markers, test doubles and stub
implementations, so the loss concentrates in test and adapter layers rather than
being spread evenly.

On a 2,669-node Rust/TypeScript repository, re-indexing after a local fix
recovered 13 structs and 9 implements edges — every one of them a unit
struct such as SystemClock, StubTransport, MockEncoder, StubSettings.
Asking for implementors of a trait, or the impact radius of one, silently omitted
these.

Cause

extractStruct treats a missing body field as a forward declaration and returns
before minting the node. That is correct for C/C++ (struct Foo; really is a
forward declaration) but Rust has no forward declarations — a bodiless struct
is always a complete definition.

This looks like the same case two earlier patches already recognised:

• C# Records are not Indexed #831 carved out C# positional records (record struct M(decimal Amount);)
  as "complete definitions with no body block".
• C++ forward declarations (class Foo;) index as phantom class nodes, masking the real definition #1093 added skipBodilessClass so a bodiless class is kept unless a
  language opts into skipping it.

Struct kept the unconditional skip, so Rust was never covered.

The kernel's Rust walker mirrors the behavior deliberately — it is listed in
rustlang.rs's bug-for-bug quirk header and in the R7b port checklist — so both
walkers need the same change to stay at parity.

I have a fix ready and will open a PR referencing this issue.