add opt-in post-display print hooks

docs/plugin-api-additions/post.md

@@ -0,0 +1,21 @@
+# Post-display print observers
+
+Python: `hook_print_after(name, callback, userdata=None, priority=PRI_NORM)`.
+The callback is `(word, word_eol, userdata, attrs)`, matching hook_print_attrs().
+It runs after display_event() has handled the text, so prnt() follows the original
+text. Return values are ignored. Events eaten by a pre-hook or hidden by client
+preferences do not invoke these observers. This is post-display notification,
+not a notification that the entire incoming IRC command has finished updating state.
+
+C: `zoitechat_hook_print_after(ph, name, priority, flags, callback, userdata)`;
+flags must be zero. Check `get_info("api_hook_print_after")` for "1" before
+accessing the appended Windows function-table slot. Existing pre-hook timing is
+unchanged. Unhook/unload use existing lifecycle functions. Recursive emission
+skips an already-running observer to prevent it immediately invoking itself.
+
+```python
+def after(word, word_eol, userdata, attrs):
+    zoitechat.prnt('This follows the original channel message')
+
+zoitechat.hook_print_after('Channel Message', after)
+```