docs: author the sub-category taxonomy and C0 explanation

docs/manual/08-components-reference/index.md

@@ -18,3 +18,14 @@ category variable.
     C4         [Integrations](/08-components-reference/04-c4-terminal-and-tool-integration/) — Kitty/Wezterm integrations, starship hooks, fzf theme
     C5         [Logging and Capture](/08-components-reference/05-c5-logging-and-capture/) — Session logs, command duration
     C6         [Greeting & First-Run UI](/08-components-reference/06-c6-greeting-and-first-run-ui/) — Custom startup banner
+
+## Per-function overrides: `C0`/`always`
+
+Every guarded function or file can also carry a reserved `always/on` or
+`always/off` tag in its `# COMPONENT` header, independent of every C1-C6
+category and sub-category toggle and invisible to `config-settings`. An
+`always/off` tag disables that function unconditionally; an `always/on`
+tag enables it unconditionally, ignoring the state of every other tagged
+sub-category. This is a per-function escape hatch for cases too granular
+or too idiosyncratic to justify a taxonomy entry -- edit the header
+directly and run `__fish_config_op_registry_rebuild` to apply the change.