testing: add bloom-storm chaos scenario

Six-node depth-4 mesh with an induced upstream parent flap. Asserts a
trailing-window ceiling on per-node `stats.bloom.sent` and a sanity
floor on parent-switch count over a ~3-4 min observation window.

Guards against the regression class where a spanning-tree update that
changes only an internal path edge (no root or depth delta) fails to
be properly contained and instead propagates to leaves as a sustained
bloom-traffic oscillation, visible only at fleet scale and only after
several minutes of uptime.

Adds a new chaos primitive (`link_swap`) for deterministic asymmetric
link-cost flapping and a post-run assertion framework with two
checks:

  - `bloom_send_rate.max_per_node`: trailing-window ceiling on the
    `show_bloom` stats counter delta. Calibrated against the
    post-mortem reproduction harness data (per-variant counter table
    against pre-fix vs post-fix binaries).

  - `min_parent_switches.min_total`: sanity guard against a
    misconfigured harness where the flap inducer fires but the
    topology never produces a real parent-switch event (e.g., wrong
    root election from a different seed). Without this, the
    bloom-rate assertion would trivially pass on any binary
    including a regressed one.

The runner exits 3 on assertion failure (alongside 0 success and 2
panic-detected). Threshold derivation is documented in the scenario
README; the seed pin is also documented there since smallest-NodeAddr
root election is sensitive to the pubkey hash ordering.

Wired into ci-local.sh's chaos pool and the GitHub CI chaos matrix.

.github/workflows/ci.yml

@@ -366,6 +366,9 @@ jobs:
           - suite: congestion-stress
             type: chaos
             scenario: congestion-stress
+          - suite: bloom-storm
+            type: chaos
+            scenario: bloom-storm
           # ── Sidecar deployment ──────────────────────────────────────────
           - suite: sidecar
             type: sidecar