Group LSP parameter navigation on language rail

KEYMAP.md

@@ -327,6 +327,7 @@ Tool operations are first-class commands, not hidden side effects.
 | --- | --- |
 | `Space l` `h` | hover: compact card first, expandable panel for full text |
 | `Space l` `s` | signature help: active parameter first, expandable panel for overloads/docs |
+| `Space l` `n` / `Space l` `p` | next/previous parameter or argument placeholder; `g a`/`g A` are attached-keyboard accelerators for the same intent |
 | `Space l` `a` | code actions for cursor/selection/diagnostic |
 | `Space l` `r` | rename |
 | `Space l` `o` | organize imports |
@@ -368,8 +369,10 @@ editor space, and they should not make diagnostics or signature context
 unreachable.
 
 Signature help follows the same rule: show the active overload and parameter
-first; move between parameters with the parameter navigation grammar; expand only
-when the user asks to read all overloads or full docs.
+first; move between parameters with the language rail (`Space l n`/`Space l p`)
+so hover, signature, and argument navigation are discoverable together. The Go
+rail keeps `g a`/`g A` as attached-keyboard accelerators for the same movement.
+Expand only when the user asks to read all overloads or full docs.
 
 ### Multiple providers