milestone 21: abp list exceptions and a regex rule kind

src/filter/parsers.zig

@@ -22,10 +22,15 @@ pub const Kind = enum {
     domain,
     /// `text` holds one candidate suffix; every proper subdomain of it matches.
     wildcard,
-    /// A regex rule. Counted, skipped, never compiled (PLAN §2.2).
+    /// A regex line in a downloaded list. Counted, skipped, never compiled: the
+    /// engine exists for rules the operator wrote, not for lists (PLAN §2.2).
     regex,
+    /// `text` holds one candidate name an ABP exception rule (`@@||x^`) lifts:
+    /// the name itself and every subdomain of it. Only the ABP parser emits it.
+    exception,
     /// Syntactically a rule of this format, but one nxdns cannot honour:
-    /// an ABP modifier list, an exception rule, element hiding, a scheme anchor.
+    /// an ABP modifier list, an exception form outside `@@||x^`, element
+    /// hiding, a scheme anchor.
     unsupported,
 };
 
@@ -33,8 +38,11 @@ pub const Line = struct {
     kind: Kind,
     /// Borrowed from the caller's line. Not lowercased, not validated.
     text: []const u8 = "",
-    /// `.wildcard` only. ABP `||x^` covers `x` itself as well as its subdomains,
-    /// so the compiler emits an additional `.list` entry when this is set.
+    /// `.wildcard` and `.exception` only: the rule covers the anchored name
+    /// itself as well as its subdomains, which is what ABP `||x^` and `@@||x^`
+    /// mean. The compiler acts on it for a `.wildcard` line, by emitting an
+    /// additional `.list` entry; an `.exception` line needs no second entry,
+    /// because the allow walk tests the full name as well as its parents.
     covers_apex: bool = false,
 };