Adds `agent_shell_dna` cube parsing the new agent-shell::<p>:<m>:<c>::<scope>::<body>-<nonce>
format emitted by keisei-marketplace/src/lib/cryptoid.ts::agentDna.
Companion to legacy 4-segment `dna_class` (untouched per RULE Don't-Rewrite).
Accepts both 8-hex (legacy) and 16-hex (current) lengths for forward-compat.
- new file: src/agent_shell_dna.rs (235 LOC, 13 tests all pass)
- lib.rs: pub mod agent_shell_dna + module doc
Closes HIGH-2 (dna-three-layer audit).