Implement executable Forge diagnostic lifecycle

[epi13]

What changed

• add validated compact views and shared immutable storage for transition, pair, tabular, trace, graph, and composite snapshots
• add the bounded MNEL-side Forge lifecycle: explicit verifier declarations/registry, preconditions, proposal-bound probes, diagnostic-only witnesses, reference verifiers, registered mutations, comparison, health/quarantine, coverage, learned observation events, and omitted-question candidates
• add mnel forge-reference as a deterministic local study path and add lifecycle/snapshot schemas and tests

Architectural boundary

This is an MNEL diagnostic reference surface, not a replacement for external MNCS Forge and it grants no evaluator, conformance, or promotion authority. Learned observations remain distinct from verifier witnesses; disagreement is preserved.

Validation

• Python compileall and 61 unittest tests: pass
• python -m mnel demo --workspace build/demo and ledger verify: pass
• mnel forge-reference and its ledger verify: pass; four completed witnesses, agreement comparison, registered mutation, health/coverage, and proposal-only candidates
• Rust fmt, Clippy, workspace tests: pass
• provider benchmark: pass
• Ruff and git diff --check: pass
• Joern post-check: Rust parse succeeds with same focused empty method/call query; Python frontend unavailable in the installed Joern distribution