fix(graph): stop leaking edges to excluded neighbors

[davion-knight]

summary

find_neighbors() appended an edge to the response before checking whether its other endpoint passed the same retrievability/existence checks that gate node inclusion (_neighbor_ok / _node_kind). superseded, archived, and redacted claims — and missing nodes — were correctly excluded from nodes, but the edge pointing at them still went out. a response could contain an edge whose target referenced a claim id the response itself said didn't exist.

kb.neighbors shares this code path across all three surfaces (mcp, jsonl, cli), so the leak was identical everywhere.

what changed

reordered the loop in src/vouch/graph.py::find_neighbors so an edge is only recorded once its other endpoint has been accepted into visited — either just now, having passed the gate, or already accepted in an earlier iteration.

test plan

extended test_find_neighbors_excludes_superseded_claims in tests/test_graph.py to also assert result["edges"] == [] — fails on the old code (returns the dangling edge to the superseded claim), passes with the fix.

.venv/bin/python -m ruff check src tests
.venv/bin/python -m mypy src
.venv/bin/python -m pytest tests/test_graph.py -q
.venv/bin/python -m pytest tests/ -q --ignore=tests/embeddings

all clean. note: the full suite has 3 pre-existing order-dependent failures (test_fsck_clean_kb_prints_clean_and_exits_zero, test_search_fts5_backend_label, test_deindex_removes_fts_and_prov) that reproduce identically on a clean test checkout with no changes — unrelated to this PR, confirmed by stashing this diff and re-running.

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