feat(arch): surface Leiden community clusters in get_architecture

Wire the Leiden detector into get_architecture via a new 'clusters' aspect.
arch_clusters loads Function/Method/Class nodes + CALLS edges (capped for very
large graphs), runs cbm_leiden, and reports the top-N communities compactly —
each with a dominant-package label, member count, cohesion (internal vs boundary
edge ratio), representative top nodes (by degree), and the packages spanned.
Singletons are skipped to avoid noise. The mcp.c serializer already emitted the
clusters array; it is now populated.

Tests: arch_clusters_basic (two cliques + a bridge -> >=2 connected clusters).
Generous smoke coverage added for the full Cypher surface (labels/type/id/keys/
properties/size/reverse/replace/left/coalesce/substring, NOT EXISTS dead-code,
CASE, unsupported-function error) plus the get_architecture clusters aspect.