fix(typecheck): resolve indexed writes at the field path they target

[wolfy-j]

Defect

An element assignment through a record field path later in a function (r.kinds[1] = x) makes every earlier read of r.kinds on that flow version resolve to the enclosing record type. Reproduction: a library exporting type Report = {kinds: {string}, sources: {string}} and describe(): Report; a caller doing

local described = lib.describe()
same(described.kinds, {"a", "b"})   -- error: argument 1: expected string[], got Report
described.kinds[1] = "changed"

Removing the assignment, or assigning through a local, removes the error.

Cause

Flow widening for a dynamic index write resolved the container from the path's root symbol instead of the path itself, so a.b[k] = v computed a widened type from a and stored it under the path key for a.b.

Fix

• types/flow/transfer.go: processIndexerAssignmentReturnKey resolves the container type at the full path; predecessor joins are keyed by the path's segments; declaredTypeAtPath walks segments through the root declaration; mergeMapValueDomain keeps a declared map value domain from unioning with its own refinement.
• Regression tests: TestIndexedFieldElementWriteKeepsFieldType, TestIndexedFieldElementWriteKeepsFieldTypeAcrossModules, TestDeclaredMapFieldIndexerWriteKeepsValueType.

go test ./... and go vet clean. Found while linting Bee (Wippy application) under wippy lint; the runtime pins go-lua v1.5.19 and needs a release plus bump to pick this up.