feat: add deterministic content envelopes (DSE-715)

[ernestprovo23]

What changed

• add strict immutable ContentEnvelopeV1 models and RFC 8785 codec
• domain-separate every digest and bind exact content, source, transform, bundle, and parent evidence
• enforce explicit untrusted state, monotonic registered taint, bounded one-hop lineage, and code-only errors
• close model/subclass/transform-context forgery paths with exact-type and registry validation
• add exact cap/cap+1, malformed-input, property/fuzz, planted-secret, and authority-denial tests
• synchronize the Agent Trust Kernel, system context, documentation index, and implementation plan

Why

DSE-715 implements the evidence-only content/provenance foundation required by the Agent Trust Kernel. It grants no authority and does not claim whole-kernel conformance; PDP/PEP artifact enforcement and durable signed receipts remain DSE-716/DSE-717.

Validation

• focused content-envelope unit suite: 84 passed
• deterministic full fuzz suite: 43 passed
• Ruff: all checks passed
• MkDocs strict build: passed
• Mermaid render: passed
• broad coverage run: 687 passed, 2 locally diagnosed harness failures, 86.02% coverage
  • stale traceback/source mismatch rerun fresh with coverage: passed
  • sandbox process-table child-reap test rerun outside sandbox: passed
• independent CSO final verdict: SHIP (92 focused checks)
• final Conclave adversarial review: no blocker; request 7cf826994e8540faac551b5fb921a8c4

GitHub Actions is the authoritative clean-environment gate before ready/merge.

Refs DSE-715