CB-589: document that weighted placement is not cheapest-first

The weight ratio does not express a preference order. weighted spreads
spawns across every profile with a free slot, so paid spawns happen
while the free box is idle - and a profile at maxLoad freezes its score,
so it can lose the next pick after a slot frees.

The real fix is a cost-first policy (CB-589). This documents the
workaround and its trap next to the key, because bridged.yaml is
gitignored: a fresh host starts without the workaround and quietly pays,
with nothing to tell the operator why.

Comments only. BridgedConfigTest: 85 tests, BUILD SUCCESS.

bridged/bridged.example.yaml

@@ -275,6 +275,26 @@ profiles:
   #     argv: ["opencode"]
 # How an unqualified spawn chooses a profile: fixed (default, reproduces pre-CB-518 behaviour),
 # round-robin, or weighted. Omitting this key is a strict no-op for existing configs.
+#
+# `weighted` IS NOT "cheapest first" — read this before you set weights (CB-589).
+# It is smooth weighted round-robin: it spreads spawns across EVERY profile that has a free slot,
+# in weight ratio. It has no idea which profile costs money. So with local:10 / paid:2 you do not
+# get "use local, overflow to paid" — you get roughly one spawn in six going to the paid profile
+# while the local box still has a free slot.
+#
+# There is a sharper second effect. The policy's running score map lives for the daemon's whole
+# life. While a profile is at maxLoad it is filtered out and its score FREEZES, so the paid
+# profiles keep accumulating against it. When the local slot frees up it returns with a stale
+# score and can LOSE the next pick — a paid spawn while the free box sits idle.
+#
+# Until a real cost-first policy exists, the workaround is to make the ratio decisive rather than
+# proportional: give the free profile a weight so large that it wins every pick it is eligible
+# for, and paid profiles only ever take genuine overflow. On this host that is local weight 100
+# against paid weights of ~1.
+#
+# The gotcha with that workaround: it expresses a PREFERENCE ORDER through a RATIO knob. Add a
+# future profile at weight 150 and it silently outranks the free box, with nothing to warn you.
+# Re-check the weights whenever you add a profile.
 placement: weighted
 
 # How long a credential sits out after a BACKEND_EXHAUSTED classification (CB-578 stage B), in