DADBEAR unit17: collapse watcher event writes

[adamlevineagent]

Summary

• move legacy file-mutation observation mapping into the canonical observation_events helper
• remove watcher::write_mutation so non-watcher paths no longer depend on watcher to enqueue DADBEAR work
• preserve existing observation source/event labels while repointing watcher, scan rename, startup reconciliation, and rename_edit recheck call sites

Verification

• cargo test file_mutation_helper_maps_legacy_names_to_observation_events -- --nocapture
• cargo test edge_survives_endpoint_supersession -- --nocapture
• cargo test faq_link_survives_related_node_supersession -- --nocapture
• cargo test targeted_evidence_resolves_real_source_not_placeholder -- --nocapture
• cargo test phase0_stale_fail_safe_tests -- --nocapture
• cargo test test_set_file_content_hash_preserves_node_ids_order_independent -- --nocapture
• cargo test --no-run
• git diff --check