atperson: split graph.c into single-concern modules

atperson d232d3d — the second split under issue #42, closing out the graph.c refactor.

Before / after

src/core/graph.c was the largest core file at 1223 lines, mixing hash indexes, node interning, edge observation, observe walks, budget-ceiling checks, episodic-memory capture, recall, query, association ranking, and lifecycle helpers. It is now five files, each owning one concern and all comfortably under the 500-line ceiling from the modular-file mandate:

Module	Lines	Concern
graph.c	169	construction, lifecycle, RNG, config, stats, status strings, ledger-entry accessors
graph_index.c	196	open-addressing node/edge hash indexes, atp_find_node, atp_find_edge
graph_store.c	238	node interning, edge observation, array growth (atp_reserve_*)
graph_observe.c	321	observe walks, budget-ceiling dry-run, episodic-memory capture
graph_query.c	316	query tokenization, recall, association ranking, familiarity lookup

Discipline enforced

• Verbatim extraction. No logic, signature, comment, or behaviour change — the split is purely organizational. The budget check that rejects whole observations before any mutation, the eviction tie-breaks, the splitmix64 edge-key finalizer, and every walk struct land in the module their concern owns.
• Internal interface. internal.h was the only header changed, and only to raise four index helpers from static to cross-TU visibility (atp_node_index_maybe_grow, atp_node_index_insert, atp_edge_index_maybe_grow, atp_edge_index_insert) plus their existing prototypes. TU-local structs (atp_observe_walk, atp_budget_walk, atp_memory_walk, atp_query_walk, atp_recall_match, atp_ranked_edge, atp_lookup_walk, atp_token_count) stay file-private.
• Banner comments travel. The /* -------- hash indexes (issue #9) -------- */ and /* -------- episodic memory -------- */ section markers attach to graph_index.c and graph_observe.c respectively.
• Build/test green. Core (GCC + Clang), ASan/UBSan, and network builds all pass — 31/31 tests on the merged main (including the concurrent action-termination inspection-traces work, integrated via a clean rebase onto upstream).

Next

src/core/persistence.c (1037 lines) is the last file over the ceiling — its encode/decode/migration sections split next.