PTR-004: graf/ DAGParents subtraction, weave_set ptr+len

The 2026-06-15 audit found graf/ builds slices via the API then rips them apart by pointer subtraction (systemic in the DAG-parents path), passes hot R-set kernels as ptr+len, and reinvents sha1FromHex. Behavior-preserving, one worktree; blame/log/weave outputs stay byte-identical (GRAFblameident).

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