feat: liveness probes, recovery strategies, and skip_if rename

[peter-adam-dy]

Summary

• Adds k8s-inspired readiness and liveness probes for both command and start_server nodes to detect unhealthy state (dropped SSH tunnels, crashed processes, unreachable databases)
• Adds recovery strategies (notify, restart_node_and_dependents, restart_environment) configurable at project/node/variant level
• Renames verify to skip_if for clarity (alias preserved)
• Bumps schema version to "2" (v1 fully backward compatible)
• Daemon automatically detects liveness failures and triggers recovery via veld restart subprocess

New config shape

{
  "schemaVersion": "2",
  "recovery_strategy": "restart_node_and_dependents",
  "nodes": {
    "database": {
      "variants": {
        "dblab": {
          "type": "command",
          "command": "./scripts/dblab/start.sh veld-${veld.run}",
          "on_stop": "./scripts/dblab/stop.sh veld-${veld.run}",
          "outputs": ["DATABASE_URL"],
          "skip_if": "./scripts/dblab/verify.sh veld-${veld.run}",
          "probes": {
            "liveness": {
              "type": "command",
              "command": "pg_isready -h ${DB_HOST} -p ${DB_PORT}",
              "interval_ms": 5000,
              "failure_threshold": 3,
              "max_recoveries": 3
            }
          }
        }
      }
    }
  }
}

Key changes

Area	Changes
Schema	probes (readiness + liveness), recovery_strategy, skip_if, LivenessProbe def
Config	RecoveryStrategy enum, ProbesConfig, LivenessProbe, resolve_recovery_strategy(), readiness_probe(), liveness_probe()
State	NodeStatus::Unhealthy, RunStatus::Recovering, liveness tracking fields
Orchestrator	Uses probes.readiness (fallback to health_check), readiness probes for command nodes, skip_if rename
Daemon	Liveness monitor with per-probe intervals, failure counting, recovery triggering, veld restart subprocess
Graph	get_dependents() for recovery subgraph identification
Recovery	New module: stop_subgraph(), state helpers, strategy resolution

Test plan

• All 121 existing + new tests pass (cargo test)
• Zero clippy warnings (cargo clippy)
• Schema v1 configs still load (backward compat)
• verify alias works for skip_if
• probes.readiness takes precedence over health_check
• Recovery strategy resolution: variant > node > global > default
• get_dependents() correctly identifies transitive dependents
• Manual test: command node with liveness probe detects failure and triggers restart

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