Phase 3d: declaration modifiers become @annotations (Rule A)

edge/pure/export prefix keywords retired; modifiers are now leading @annotations
(@export, @edge, @pure, @deterministic). parse_one_decl collects a leading @anno
run and @export sets the fn export flag; dead edge-dispatch removed from
parse_system. Migrated examples/lib/combat.ludic; old prefixes now rejected.

Behavior-identical: the export flag is parse-only in the self-hosted emitter
(emits @fn_<name> for every fn, never reads the flag), so IR is byte-identical.
Reseeded, fixpoint holds, test.sh 14/14. Docs updated. Phase 3 complete.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

selfhost/parse_game.ludic

@@ -20,7 +20,6 @@ fn skip_clause() -> void {
 }
 
 fn parse_system() -> Node {
-  if is_id("edge") { pi = pi + 1 }             # optional `edge` modifier before `system`
   pi = pi + 1; let n = node(N_SYS); n.s = eat_id(); n.ty = "Update"
   # clauses: @anno, phase X, reads/writes/needs/uses [..], query (..) [..]
   # A `query (vars) [terms] where c` clause desugars to a body wrapped in one