docs: add comprehensive docstrings and inline comments

Add docstrings to all handler functions in daemon/handlers/firewall.py, covering
params, return values, and raised exceptions. Add inline comments to
_config_apply() reconciliation steps and the request body merge order.

Add docstrings across lib/ modules for emit helpers (_emit_str, _emit_int, etc.),
volatile stripping logic, two-layer diff strategy, sync event dispatch, and all
cross-subsystem sync subscribers (DnsToFirewall, WgToFirewall, FirewallToDhcp,
NetworkToAllSync).

Document WireGuard/networkd config parsers and key-value mappers in
system_import.py. Add docstrings to _ep(), Registry.decorator,
setup_logging, and _replace helper across daemon/ and lib/.

daemon/client.py

@@ -94,6 +94,15 @@ def _format_path(path: str, params: dict[str, Any] | None) -> str:
         return path
 
     def _replace(m: re.Match[str]) -> str:
+        """Regex callback that replaces ``<key>`` segments with URL-encoded values.
+
+        Args:
+            m: Match object containing the parameter name.
+
+        Returns:
+            URL-encoded value from params dict, or original text if key
+            not found.
+        """
         key = m.group(1)
         if key in params:
             return urllib.parse.quote(str(params[key]), safe="")