webui: graph tools parity and documentation

[IceRhymers]

Part of #82. Depends on #86 and #87.

Summary

Web UI parity for graph queries, plus user-facing documentation for the edge model and tools.

Scope

• Web UI surface (references/imports lookup from a result or symbol) using the same app/service.py payload builders — no duplicated graph logic (negation/branch-chips parity precedent).
• Parity tests between MCP and webui payloads.
• Docs: README section for the graph tools; edge-model explanation ("raw edges, query-time resolution, candidate sets" — including why results are candidate sets); reindex runbook note for the INDEX_SEMANTICS_VERSION bump.

Acceptance criteria

• Web UI can run find-references and imports lookups with candidate-set presentation.
• Parity test proves MCP and webui serve identical payloads.
• README + runbook updated; docstring/docs language consistent about candidate semantics.

[IceRhymers]

Completed via child PR #95, squash-merged into the #82 integration branch (integration/knowledge-graph-reference-edges). The umbrella PR #89 is now entering final integration validation and protected-master review.