Refresh player with native block editor preview and local assets

Preserve shortcode rendering, normalize options, bundle the player locally,
and add reproducible packaging and WordPress integration tests.

tests/fixtures/hello.cast

@@ -0,0 +1,4 @@
+{"version":2,"width":64,"height":12,"timestamp":1700000000,"env":{"TERM":"xterm-256color","SHELL":"/bin/sh"},"title":"Local integration fixture"}
+[0.0,"o","$ printf 'Hello from wp-asciiplayer'\r\n"]
+[0.1,"o","Hello from wp-asciiplayer\r\n"]
+[12.0,"o","$ "]

tests/fixtures/malformed.cast

@@ -0,0 +1 @@
+This is deliberately not an asciicast recording.