test(ingestor): pin three behaviours left unpinned after #190 (failed tick, NULL route type, originator exclusion)

[dborup]

Summary

These are test gaps left after #190 (observer anchor in the ingestor). They come from the independent review of head 0f4c167a: three of the reviewer's five mutants survive the whole cmd/ingestor suite. The code is believed correct. The tests just do not pin these behaviours. Low priority.

Relates to #188, #190.

Surviving mutants

#	Mutant	What is unpinned
X1	A neighbour-edge build tick that fails still marks the graph as a post-build snapshot	The backfill readiness gate (resolvedPathBackfillReady) must not treat a failed build as complete. A test should fail a tick (for example a DB error) and assert that the backfill does not advance.
X3	Rows with a NULL route_type are not skipped by the flood-only observer↔last-hop edge rule	The builder adds the observer edge only for route types 0 and 1. A NULL route type must not create an edge. There is no test for the NULL case.
X4	The originator is not excluded in the backward chain	resolvePathBackward should never resolve a hop to the packet's originator. No test covers it.

Acceptance criteria

• One test per mutant. Each test is red with the mutant applied and green on the code.
• No production code change unless a test reveals a real defect. If it does, describe the defect in the PR.

[dborup]

Fixed by #207 (merged as 6bdaa8f7). Each of the three behaviours left unpinned after #190 now has a test that is red with the mutant and green on the code:

• X1: a failed edge-build tick does not mark the graph ready;
• X3: a NULL route_type does not create an observer edge;
• X4: the originator is excluded from the backward chain.

The review nit (the X3 test repeats the seedObservations190 helper) is cosmetic and was not carried forward.