test(code-graph): verify distinct FQNs for multiple init() functions

Adds a fixture for Go's special init() and main() functions to the code-graph integration test suite.

The fixture verifies that main() is extracted as a Function definition, that two init() functions in the same file both produce distinct Definition nodes (distinct by byte-range derived id, not FQN), and that both share the package-scoped FQN main.init — which is the correct Go behavior. The fixture is registered in suites.rs and runs as part of the standard integration test suite.

Closes #729 (closed)

Ran mise run test:integration:codegraph locally — 172 tests passed including the new fixture.

This MR does not introduce any performance regression.