health: exclude mocked module edges from static test reachability

[BartWaardenburg]

Problem

Static coverage reachability follows an import into a module that Vitest replaces with vi.mock().
The mocked module and its exports are therefore reported as test-covered even though the real module
is never executed by that test.

This also changes coverage-derived health severity when a wrapper test is added, which can make
stable health identities regress unexpectedly.

Minimal reproduction

// src/dependency.ts
export const renderDependency = (): string => "real dependency";

// src/wrapper.ts
import { renderDependency } from "./dependency";

export const renderWrapper = (): string => renderDependency();

// src/wrapper.test.ts
import { describe, expect, it, vi } from "vitest";
import { renderWrapper } from "./wrapper";

vi.mock("./dependency", () => ({
  renderDependency: vi.fn<() => string>(() => "mocked dependency"),
}));

describe("renderWrapper", () => {
  it("uses the mock", () => {
    expect(renderWrapper()).toBe("mocked dependency");
  });
});

With src/entry.ts as the package entry, run:

fallow health --coverage-gaps --format json --quiet --no-cache

Fallow 3.9.1 reports three runtime files, two covered files, and only src/entry.ts as untested.
The two covered files are src/wrapper.ts and src/dependency.ts.

Expected

The static coverage graph should treat the literal target of vi.mock("./dependency") as a mocked
edge for that test root. src/wrapper.ts is test-reachable, but src/dependency.ts is not covered
by this test.

The same likely applies to jest.mock() and vi.doMock() where the target is a static string.