DOG-013 MED: dog/ reuse — dup decoders/walks, hand-rolled maps, boilerplate

The review found repeated hand-rolled machinery across dog/ that abc already provides or that should be factored once. This is behaviour-preserving cleanup; it also de-risks the security tickets, since DOG-008's index-validation fix has to land in the three duplicate WEAVE decoders unless they are consolidated first. Distinct from PTR-002 (which is the pointer-arithmetic/buffer-poke style pass) — this is structural duplication and missed abc reuse (CLAUDE.md #13). Method: Issues.

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