test(09-02): add Playwright UX-02 port autofill test

- Create tests/e2e/test_port_autofill.py - headless chromium test for PRNT-03
- Verifies IP->port_name auto-derivation via Alpine @input handler at /printers
- Update 09-VALIDATION.md: mark UX-02 tasks green, cite test file as evidence

tests/e2e/test_port_autofill.py

@@ -0,0 +1,29 @@
+"""UX-02: live-browser verification of PRNT-03 Alpine IP->port auto-derivation."""
+from __future__ import annotations
+
+import pytest
+
+
+def test_port_autofill(page, live_server: str) -> None:
+    """Typing an IP address into the printer form auto-populates port_name.
+
+    Evidence for UX-02: tests/e2e/test_port_autofill.py
+    Closes: PRNT-03 Alpine.js port auto-derivation requirement.
+    """
+    # /printers renders a full page (extends base.html) with Alpine.js loaded
+    # and the printer form embedded via {% include "partials/printer_form.html" %}
+    page.goto(f"{live_server}/printers", wait_until="domcontentloaded")
+
+    # Wait for Alpine to initialise (x-data hydration on the form wrapper)
+    page.wait_for_selector("input[name='ip_address']")
+
+    # Fill triggers the 'input' event that Alpine @input listens to
+    page.fill("input[name='ip_address']", "192.168.1.100")
+
+    # Alpine @input reacts synchronously; wait_for_function keeps the test stable
+    page.wait_for_function(
+        "document.querySelector(\"input[name='port_name']\").value === 'IP_192_168_1_100'",
+        timeout=2000,
+    )
+
+    assert page.input_value("input[name='port_name']") == "IP_192_168_1_100"