feat(core,ledger): decision mutation contract B1+B2 (GH-330)

[fagemx]

Summary

• B1 (Status Sync): Add status_to_is_active() helper enforcing COMPAT-01 invariant. append_event() and insert_imported_decision() now write both status and is_active in sync. Deactivation UPDATE sets status = 'superseded' alongside is_active = FALSE.
• B2 (DecisionPayload fields): Extend DecisionPayload with 5 optional fields (authority, affected_paths, tags, review_after, reversibility). Decision materialization reads these from payload with safe defaults. extract_decision() parses new fields from event JSON.
• All existing callers updated to compile with new struct shape (explicit None for new fields).
• Pre-existing clippy warnings fixed in edda-derive and edda-serve test modules.

Contract compliance

• COMPAT-01: is_active and status always agree — verified by integration test
• MUTATION-01: All UPDATE decisions statements are in sqlite_store.rs only
• CLIPPY-01: cargo clippy --workspace --all-targets -- -D warnings passes

Test plan

• test_status_to_is_active — unit test for the mapping helper
• test_status_is_active_sync_on_insert — insert + supersede cycle with COMPAT-01 check
• test_decision_payload_new_fields_roundtrip — write with all new fields, read back correctly
• test_decision_payload_defaults_when_none — verify defaults (human/[]/[]/NULL/medium)
• test_decision_payload_serde_backward_compat — old JSON without new fields deserializes
• test_decision_payload_serde_with_new_fields — new JSON round-trips correctly
• cargo clippy --workspace --all-targets -- -D warnings zero warnings
• cargo test -p edda-core -p edda-ledger all pass (2 pre-existing failures on main excluded)

Closes #330

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