feat(graph): deterministic Louvain community detection (hq-zlph)

[scbrown]

What (aegis-1p0 Gap 3)

Finishes the planned community detection on quipu's existing petgraph Projection API. New tool_project algorithm louvain (alias community) that detects modularity-based communities and can persist them as queryable facts.

Implementation

• graph::louvain — hand-rolled, pure-Rust deterministic Louvain (modularity local-moving phase). The projected directed multigraph is read undirected/weighted (parallel edges summed, self-loops dropped). Determinism is a hard AC: ascending entity-id visitation, ascending-label candidate evaluation, lowest-label tie-break → the same graph always yields the same partition, independent of hash order. No C-backed clustering crate (quipu = zero-unsafe/minimal-dep). Returns groups + Newman–Girvan modularity.
• tool_project now takes &mut Store (handler ro_handler! → rw_handler!); read-only by default, persist:true opt-in.
• persist_communities — writes quipu:memberOfCommunity via the store write path with provenance source = "algo:louvain". Bitemporally supersedes the prior derivation (AC 5): reconciles by (entity, community-term), retracting changed memberships and asserting new ones, so a current-facts query always returns exactly the latest partition — no stale accumulation. (Diff rather than retract-all+reassert because the facts table is keyed (e,a,v,tx) — retracting and re-asserting an identical triple in one tx would collide.)
• Not an access boundary (AC 6): emergent clustering, distinct from group_id; documented in the module header, the persist fn, and the design doc so no consumer builds access control on it.

Design provenance

Scope + write-path semantics resolved with dearing (opt-in persist, store write path) and ian-endorsed (AC 5 supersede-on-rerun, AC 6 not-an-access-primitive). The bead's original "persist like quipu:pageRank" was corrected — tool_project arms were all read-only; this adds the first write path deliberately and opt-in.

Tests (all green on both CI feature sets)

• louvain_finds_two_clusters_deterministically — two-clique fixture separates correctly; same graph → identical partition (AC 1/3).
• louvain_persist_writes_queryable_membership — persist:true writes SPARQL-queryable quipu:memberOfCommunity; read-only default writes nothing (AC 2).
• persist_supersedes_prior_membership — re-run with changed partition leaves exactly one active membership per entity (AC 5).

cargo fmt --all --check, clippy -D warnings, and full tests pass under --no-default-features and --features shacl. No new MCP tool (algorithm param on existing quipu_project), so no tool-count change.

Docs: pagerank.md community section, mcp-tools.md reference.

Closes hq-zlph. (quipu in hq rig until qp- stands up.)

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