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Fix weight-slot leak, clear_edges OOB, and vertex overflow in adapt_sgraph (#1182)

Three defects in AdaptGraph (src/crab/splitdbm/adapt_sgraph.hpp):

1. forget() leaked in-edge weight slots. Each edge k -> v owns one _ws slot
   referenced from both _succs[k] and _preds[v]. The in-edge loop removed v
   from _succs[k] without recycling that slot into free_widx, so _ws grew
   monotonically across the many havoc/assign/forget operations a verification
   performs. Iterate _preds[v] as key/value and push the slot to free_widx,
   mirroring the out-edge loop. (Self-loops are recycled once via the out-edge
   loop, which first removes v from _preds[v], so no double-free.)

2. clear_edges() cleared _ws but left free_widx populated, so the next add_edge
   would take a stale index and write _ws[idx] into an emptied vector (OOB).
   Clear free_widx too. (Currently no callers, but fixed rather than left armed.)

3. verts()/new_vertex() cast the vertex count to VertId (uint16_t); at 65536
   vertices this wraps to 0. Far beyond any eBPF program, but add an assert.

Signed-off-by: Elazar Gershuni <elazarg@gmail.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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