test(relay): rename misnamed precedence test; document the flat-key fallback nuance

test_flat_key_wins_over_subblock asserted the OPPOSITE of its name (the
sub-block wins, matching _relay_slack_extra). Rename to what it proves.
Also note in _resolve_cron_surface_mode why its fallback differs from
_relay_slack_extra's all-or-nothing sub-dict: the flat key is the legacy
staging shape, and a flat knob applies to every fronted platform, gated
only by the per-platform D6 capability check.

cron/scheduler.py

@@ -1510,6 +1510,15 @@ def _resolve_cron_surface_mode(pconfig, logical_platform_name: str) -> str:
       scoped to its logical platform so a ``slack:`` block cannot leak onto
       another fronted platform.
 
+    Precedence nuance vs _relay_slack_extra: that helper is all-or-nothing
+    (a sub-dict REPLACES the flat extra entirely), while this one falls back
+    to the flat key when the sub-block exists but omits the knob. The
+    difference is deliberate — the flat key is the legacy staging shape and
+    must keep working — but note a flat ``cron_continuable_surface`` then
+    applies to EVERY platform this relay fronts; only the per-platform D6
+    capability gate contains it. Scope the knob under the sub-block on
+    multi-platform relays.
+
     Field gap (2026-08-18): the scheduler read only the flat key, so on the
     relay lane — where pconfig is platforms.relay — operators had NO working
     location for the knob and briefs always threaded.

tests/relay/test_relay_inchannel_continuable.py

@@ -169,9 +169,10 @@ class TestSurfaceKnobResolution:
         pconfig = SimpleNamespace(extra={})
         assert _resolve_cron_surface_mode(pconfig, "slack") == "thread"
 
-    def test_flat_key_wins_over_subblock(self):
-        """Legacy-fallback precedence mirrors _relay_slack_extra: an explicit
-        flat key keeps working when both are present."""
+    def test_subblock_wins_over_flat_key(self):
+        """Sub-block precedence: the per-logical-platform sub-block is the
+        documented relay shape and wins when both are present — matches
+        _relay_slack_extra (sub-dict preferred over flat extra)."""
         pconfig = SimpleNamespace(
             extra={
                 "cron_continuable_surface": "thread",