refactor(shared): move MixedChainStep into gitnexus-shared

U3. The shared package owns the ScopeExtractor output contract, and a receiver
chain is about to become part of it, so the type moves to sit beside
`ReferenceSite` — the interface that will carry it.

Type-only move. `extractMixedChain` stays in the analyzer: it walks a
tree-sitter AST, so it cannot follow. `MAX_CHAIN_DEPTH` stays with it for the
same reason — nothing shared needs the bound yet, and moving it now would be
moving it on speculation.

`call-analysis.ts` re-exports the type so its existing importers keep a single
import site. The re-export is `import type` + `export type` rather than a bare
`export type { X } from`, which creates no local binding and left the file's own
two uses of the name unresolved.

Verified: `tsc --noEmit` clean in both packages, and the receiver-resolution
gate green end to end.

(The unit suite has seven failures on this tree, all subprocess-spawning CLI
tests timing out. Three pass in isolation — load flake. The fourth,
`skip-git-cli.test.ts`, fails harder on untouched `main` in this sandbox — six
failures there against four here — so it is environmental and predates this
branch.)

gitnexus-shared/src/index.ts

@@ -83,7 +83,12 @@ export type { ResolveTypeRefContext } from './scope-resolution/resolve-type-ref.
 
 // ScopeExtractor output contracts (RFC §3.2 Phase 1; Ring 2 PKG #919)
 export type { ParsedFile } from './scope-resolution/parsed-file.js';
-export type { ReferenceSite, ReferenceKind, CallForm } from './scope-resolution/reference-site.js';
+export type {
+  ReferenceSite,
+  ReferenceKind,
+  CallForm,
+  MixedChainStep,
+} from './scope-resolution/reference-site.js';
 export type {
   CallableFlowOperand,
   CallableFlowExpectedSignature,

gitnexus-shared/src/scope-resolution/reference-site.ts

@@ -124,3 +124,18 @@ export interface ReferenceSite {
    */
   readonly argumentTypeClasses?: readonly ParameterTypeClass[];
 }
+
+/**
+ * One step in a mixed receiver chain — the decoded form of a receiver that is
+ * itself an expression rather than a bare name.
+ *
+ * For `svc.getUser().address.save()`, the receiver of `save` decodes to
+ * `[{ kind: 'call', name: 'getUser' }, { kind: 'field', name: 'address' }]`
+ * over a base receiver of `svc`.
+ *
+ * Lives here rather than beside its producer because it is part of the
+ * ScopeExtractor output contract that this package owns: the producer
+ * (`extractMixedChain`) walks a tree-sitter AST and so must stay in the
+ * analyzer, but the shape it yields crosses into resolution.
+ */
+export type MixedChainStep = { kind: 'field' | 'call'; name: string };

gitnexus/src/core/ingestion/utils/call-analysis.ts

@@ -1,3 +1,5 @@
+import type { MixedChainStep } from 'gitnexus-shared';
+
 import type { SyntaxNode } from './ast-helpers.js';
 import { CALL_ARGUMENT_LIST_TYPES } from './ast-helpers.js';
 
@@ -362,8 +364,14 @@ const FIELD_ACCESS_NODE_TYPES = new Set([
   'member_binding_expression', // C# null-conditional (user?.Address)
 ]);
 
-/** One step in a mixed receiver chain. */
-export type MixedChainStep = { kind: 'field' | 'call'; name: string };
+/**
+ * One step in a mixed receiver chain.
+ *
+ * Owned by `gitnexus-shared` — it is part of the ScopeExtractor output
+ * contract, and resolution consumes it. Re-exported here so the producer's
+ * existing importers keep a single import site.
+ */
+export type { MixedChainStep };
 
 /**
  * Walk a receiver AST node that is itself a call expression, accumulating the