test: cover interface-extends-interface EXTENDS + entry/exit/param byte-slice fidelity

[rahlk]

Two coverage gaps from epic #26:

1. The EXTENDS path from an Interface endpoint (interface B extends A) is implemented and declared (feat: neo4j EXTENDS/IMPLEMENTS inheritance edges #33) but no fixture exercises it — a regression that dropped interface-extends resolution would pass the current suite.
2. L3 byte-slice fidelity (fix: byte-sliceable L3 statement nodes #28) is directly asserted only for statement nodes; entry/exit/param share the same emitNode offset path but have no behavioral slice test.

Add a first-party interface hierarchy to the neo4j fixture + entry/exit/param slice asserts.