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>

README.md

@@ -140,7 +140,7 @@ same highlighting, checking and formatting as the source tree.
 - `scene` / `layer` / `enter` — mutually-exclusive game states, each with
   `on enter`/`on exit` hooks and layered systems (layer order = draw order).
 - `var` — typed module-level state, included in save/load snapshots.
-- `module` + `export fn` — compile a .ludic file to a shared library whose
+- `module` + `@export fn` — compile a .ludic file to a shared library whose
   exported functions are ordinary C-ABI symbols.
 - `extern fn … = "symbol"` — call any C-ABI library, Ludic or otherwise.
 - `--target wasm32-unknown-unknown` — the same game in a browser: the runtime,