Preserve peers.toml formatting on writes

[myobie]

Summary

• update only changed peer, policy, and Git remote data with toml_edit
• preserve comments, spacing, unknown fields, entry order, and user changes to the generated header
• keep nested addressed-peer tables valid across add, update, and remove operations
• write the default header only when Fabric creates the file

Tests

• cargo test --lib (467 passed, 4 ignored measurement tests)
• cargo test --bin fabric (19 passed)
• cargo check --all-targets
• git diff --check

The format-preservation regression failed against main before the writer changed.