docs(readme): record the MAC-less discovery contract (t37 D3, t39)

t37 made MAC-less zeroconf discovery honour Home Assistant's own
`_async_handle_discovery_without_unique_id` contract — the helper is a coroutine
and its dedup (abort `already_configured` when ANY entry exists for this domain,
abort `already_in_progress` when another flow runs) only happens when it is
awaited. That is stricter than the accidental host-equality guard it replaced,
so it changes what a user can expect:

* with a MAC, discovery still aborts duplicates and updates a moved device's
  hosts;
* without a MAC, a second (or third) Shield must be added MANUALLY — discovery
  will abort as "already configured".

Someone with two Shields would otherwise read that abort as a bug. The README
now has a "Discovery (zeroconf)" section under Configuration stating the rule and
the manual route, and the drift sweep's docstring carries the same note next to
the D3 entry, so the decision is discoverable from wherever you meet it.

Verified: 289 passed, 2 skipped; ruff clean.

tests/ha_api_drift.py

@@ -47,6 +47,12 @@ interpreter floor Home Assistant itself declares lives here too
 (``HOMEASSISTANT_PYTHON_FLOOR``), where CI's guard reads it instead of repeating
 the number.
 
+Behaviour note for users (t37 D3): MAC-less zeroconf discovery now follows Home
+Assistant's stricter contract — the awaited
+``_async_handle_discovery_without_unique_id`` aborts ``already_configured`` as
+soon as ANY entry exists for this domain, so a second Shield that advertises no
+MAC is added manually. See README "Discovery (zeroconf)".
+
 Without homeassistant installed the sweep cannot resolve anything: ``scan()``
 returns a single ``no-homeassistant`` finding, the counters stay at zero (so the
 floors fail on purpose) and nothing is ever reported green. The pytest wrapper