Test cost-based parent selection and kernel-drop detection as unit tests

The cost-selection chaos scenarios (cost-reeval, cost-avoidance,
cost-stability, depth-vs-cost, mixed-technology, bottleneck-parent) tested
TreeState::evaluate_parent's decision logic through a Docker mesh that could
not exercise it reliably: the tree roots at whichever node holds the smallest
NodeAddr, MMP link costs take several measurement windows to settle, and the
parent hold-down plus hysteresis timing all confound the outcome. A
deterministic link-cost flap still produced zero periodic parent switches in a
full run.

Replace those six scenarios with deterministic unit tests in src/tree/tests.rs
that drive evaluate_parent directly: cheaper-link selection at equal depth,
switch-on-cost-change, hysteresis suppressing a marginal change while allowing
a significant one, and the depth-versus-cost effective-depth tradeoff. Each is
constructed so that breaking the cost or hysteresis logic makes it fail.

The congestion kernel-drop signal (SO_RXQ_OVFL) cannot be provoked
deterministically in Docker: a fresh daemon reader keeps up with
container-speed traffic, so the socket receive queue never overflows (an
unshaped run with a 4 KB buffer and heavy traffic recorded zero drops on every
node). Extract the drop-detection edge -- read the cumulative counter, fire an
event only on the transition into a new drop burst -- into
TransportDropState::observe_drops and unit-test it directly. congestion-stress
keeps its ECN and MMP congestion-signal assertions, which do need the real
shaped bottleneck queue.

Remove the retired scenarios from both CI runners and update the chaos README.

.github/workflows/ci.yml

@@ -412,24 +412,6 @@ jobs:
           - suite: tcp-mesh
             type: chaos
             scenario: tcp-mesh
-          - suite: bottleneck-parent
-            type: chaos
-            scenario: bottleneck-parent
-          - suite: cost-avoidance
-            type: chaos
-            scenario: cost-avoidance
-          - suite: cost-reeval
-            type: chaos
-            scenario: cost-reeval
-          - suite: cost-stability
-            type: chaos
-            scenario: cost-stability
-          - suite: depth-vs-cost
-            type: chaos
-            scenario: depth-vs-cost
-          - suite: mixed-technology
-            type: chaos
-            scenario: mixed-technology
           - suite: congestion-stress
             type: chaos
             scenario: congestion-stress