ECDT: add decision proposal boundary

[aeonmatrixcommand-debug]

Adds an explicit Decision Proposal Boundary to the Enterprise
Cognitive Digital Twin.

Architecture:

Scenario Engine
|
v
Recommendation
|
v
Decision Proposal
|
v
Validation / Authorization Boundary

Proposal Contract:

• status is PROPOSED
• deterministic proposal_id
• immutable proposal model
• evidence lineage preserved
• policy context preserved
• deterministic trace generated

Authority Boundary:

• Proposal is not authorization
• Proposal is not execution
• No authorization_id is created
• No execute/apply/deploy/promote interface
• No executor reference
• No production side effects

Validation:

• Proposal tests: 10 passed
• ECDT tests: 43 passed
• Full regression: 294 passed, 1 skipped
• git diff --check: passed
• Proposal execution interface scan: NONE

Scope:

• src/intelligence/ecdt/init.py
• src/intelligence/ecdt/decision_proposal.py
• src/intelligence/ecdt/decision_proposal_builder.py
• tests/ecdt/test_decision_proposal.py