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/.

lib/logging.py

@@ -76,6 +76,15 @@ def setup_logging(level: str | None = None) -> None:
         """
 
         def _open(self):
+            """Override to open log file with group-write permissions (0664).
+
+            Temporarily clears umask to ensure the file is writable by both the
+            WebUI process (vacuum-wall user) and daemon process (vacuum-walld user)
+            when they share a group. On existing stale files, chmod's to 0664.
+
+            Returns:
+                Open file object in append mode.
+            """
             # Ensure group-write on an existing stale file (e.g. left by the
             # other process with a stricter umask at creation time).
             with contextlib.suppress(OSError):