fix(graph): bounds-check vertex indices in insert_edge (#34)

[bushidocodes]

Summary

Fixes #34. insert_edge in src/sean/wasm/graph.c wrote g->edges[source], g->degree[source] (and the undirected mirror at destination) without any bounds check. Those arrays are fixed-size MAXV + 1 (valid 1-indexed range [1, MAXV]), so a vertex index outside that range — reachable from JavaScript via the exported insertEdge() WASM API — writes past the end of the arrays and corrupts WASM linear memory. It also set number_vertices = max unconditionally, which would later drive build_csr to allocate/iterate out of bounds.

Fix

• Add a guard at the top of insert_edge (before the number_vertices update) rejecting any source/destination outside [1, MAXV], with a stderr message and early return.
• Placement matters: guarding before number_vertices = max prevents an out-of-range index from poisoning build_csr's sizing/iteration.
• Mirrors the Issue Kruskal: fixed 200-element arrays overflow on JS-supplied edge/vertex counts #9 fix in kruskal's insertadjver. The public insertEdge() in main.c forwards straight through to insert_edge, so the single guard covers the WASM entry point.

Tests

Two Unity regression tests in test_graph.c:

• test_insert_edge_rejects_oversized_vertex — source/destination > MAXV
• test_insert_edge_rejects_nonpositive_vertex — vertex 0 / negative

Both fail red without the guard (number_vertices becomes non-zero) and pass green with it. Verified locally:

7 Tests 0 Failures 0 Ignored   # test-sean (was 5)

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