feat: report per-connection status in transport stats

[nange]

Summary

Add a new conns_status field to the client transport stats (exposed via /stats JSON and the periodic [STATS] log) describing the health of each underlying HTTP/2 connection.

The value is an array-like string, one element per live connection:

"[0:3:degraded, 1:2:expiring, 2:1:active, 3:1:heavy]"

Each element is <connection index>:<active streams>:<status>, where status is one of:

• active — no special flags (healthy)
• heavy — hosts at least one heavy stream
• degraded — persistently low throughput confirmed by the health loop/probe
• expiring — connection exceeded lifetime/bytes limit, awaiting rotation

Multiple flags are joined with + (e.g. heavy+expiring) so no state is hidden. Entries are sorted by stable connection index; an empty pool renders as []. Server-side snapshots omit the field (omitempty).

Changes

• transport/transport.go: add ConnsStatus to TransportStats (json:"conns_status,omitempty")
• transport/http2/client.go: populate the field in Stats() via new slotStatus/slotStatusString helpers
• transport/http2/slot.go: add stable idx to transportSlot (retire swap-removes scramble live order)
• cmd/easyss/main.go: log conns_status in the periodic [STATS] line
• transport/http2/slot_status_test.go: unit tests for status derivation and rendering

Tests

• go test ./transport/... ./stats/... pass
• go build ./... pass
• golangci-lint: 0 issues