feat(javascript): link assignment right-hand sides to their writes (#6206)

* feat(javascript): link assignment right-hand sides to their writes

A declaration carries its value forward, but a later assignment to a
variable, a property or through a compound operator stopped the forward
trace at the right-hand-side read. The semantic IR now records one
syntactic assignment flow per simple assignment target (ranges, operator,
owning callable), and the graph projection connects each retained read
of the right-hand side to the write occurrence through a defines
relation. Compound assignments also consume the previous value of the
target; the written value itself stays unknown, keeping the boundary of
#4342 intact.

Fixes #6193

* fix(javascript): preserve computed key assignment ownership

* test(javascript): retain nested callback assignment ownership

* docs(javascript): record assignment influence verification

* test(mcp): isolate mocks and acknowledge cancellation boundaries

* test(javascript): retain CLI diagnostics and current-main verification

---------

Co-authored-by: gensokyo-dev <342580529+gensokyo-dev@users.noreply.github.com>

src/domain/javascript/javascriptSemanticIr.ts

@@ -100,6 +100,8 @@ export interface JavaScriptSemanticCallable {
   readonly containerScopeId: string;
   readonly bodyScopeId: string | null;
   readonly location: JavaScriptSourceRange;
+  /** Evaluated by the enclosing callable, before this method is invoked. */
+  readonly computedKeyLocation: JavaScriptSourceRange | null;
   readonly returnSites: readonly JavaScriptSemanticReturnSite[];
   readonly returnCoverage: JavaScriptSemanticReturnCoverage;
 }
@@ -424,6 +426,15 @@ export interface JavaScriptSemanticModuleLink {
   readonly location: JavaScriptSourceRange;
 }
 
+/** Syntactic influence into one simple assignment target; no value evaluation. */
+export interface JavaScriptSemanticAssignmentFlow {
+  readonly target: JavaScriptSourceRange;
+  readonly source: JavaScriptSourceRange;
+  readonly sourceIsReference: boolean;
+  readonly operator: string;
+  readonly ownerCallableId: string | null;
+}
+
 /** Provider-neutral JavaScript semantic IR. */
 export interface JavaScriptSemanticIr {
   readonly schema: "JavaScriptSemanticIR";
@@ -446,6 +457,7 @@ export interface JavaScriptSemanticIr {
   readonly requestOperations: readonly JavaScriptSemanticRequestOperation[];
   readonly boundaryOperations: readonly JavaScriptSemanticBoundaryOperation[];
   readonly resourceOperations: readonly JavaScriptSemanticResourceOperation[];
+  readonly assignmentFlows: readonly JavaScriptSemanticAssignmentFlow[];
   readonly objectOperations: readonly JavaScriptSemanticObjectOperation[];
   readonly functionFingerprints: readonly JavaScriptSemanticFunctionFingerprint[];
   readonly frontiers: readonly JavaScriptSemanticFrontier[];
@@ -475,6 +487,7 @@ export const failedJavaScriptSemanticIr = (): JavaScriptSemanticIr => ({
   requestOperations: [],
   boundaryOperations: [],
   resourceOperations: [],
+  assignmentFlows: [],
   objectOperations: [],
   functionFingerprints: [],
   frontiers: [],