Refactor mode registry to factory/provider pattern and enhance confor…

[ajit-zer07]

Summary

• Mode registry refactor: Replace direct Box<dyn Mode> storage with a
  factory/provider architecture (ModeFactory, ModeDescriptorProvider,
  ModeSchemaProvider, ModeConformanceProvider traits). Modes are now
  instantiated per-call via ModeRef::factory(), eliminating shared mutable
  state across the registry read-lock boundary. Each built-in mode registers
  conformance fixture paths, exposed through all_mode_conformance().

• Strict session-start flag: Add strict_session_start to
  ModeRegistration and requires_strict_session_start() query, replacing
  the hard-coded is_standard_mode() || mode_name == "ext.multi_round.v1"
  check in runtime.rs and replay.rs. Promoted extensions automatically
  gain strict session-start enforcement.

• Extension registration validation: register_extension now rejects
  descriptors with empty message_types, blank mode_version, or
  terminal_message_types entries not present in message_types.

• Decision mode hardening: Introduce phase-gated guard methods
  (ensure_can_propose, ensure_can_deliberate, ensure_can_vote,
  ensure_not_committed) that enforce the Proposal → Evaluation → Voting →
  Committed phase machine. Evaluations and Objections are now rejected once
  voting has begun.

• Proposal mode phase tracking: Add explicit ProposalPhase enum
  (Negotiating, Converged, TerminalRejected, Committed) persisted in
  ProposalState. refresh_phase() recalculates after every state mutation.
  commitment_ready() now checks phase rather than recomputing convergence.

• Handoff mode fix: Reject HandoffContext on offers whose disposition
  is no longer Offered (e.g., already accepted/declined).

• Async storage I/O: Convert FileBackend from blocking std::fs to
  tokio::fs for atomic_write, create_dir_all, read, read_dir,
  OpenOptions, and existence checks.

• Strict recovery mode: New MACP_STRICT_RECOVERY=1 env var makes
  startup replay failures fatal instead of warn-and-skip.

• Conformance test enhancements: Fixtures now support
  expected_error_code, expected_resolution, expected_mode_state
  (subset-matched via assert_json_contains), expect_resolution_present,
  and verify_replay_equivalence. New payload encoders added for
  Evaluation, Objection, CounterProposal, Reject, Withdraw,
  HandoffDecline, and HandoffContext. New unit tests cover phase
  transitions, malformed payload rejection, and post-phase-transition guards.

Test plan

• cargo test — all existing and new unit/conformance tests pass
• cargo clippy — no new warnings
• Manual smoke test with MACP_STRICT_RECOVERY=1 to verify fatal replay
• Verify MACP_ALLOW_INSECURE=1 MACP_ALLOW_DEV_SENDER_HEADER=1 cargo run
  starts cleanly with no regressions
• Review conformance fixture JSON for correctness of expected_resolution
  and expected_mode_state assertions