Add unified circuit-breaker safety layer: guardian-bot daemon, per-contract façade, auto-trip on health degradation, and zero-liquidity guard

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Summary

This PR introduces a unified pause/circuit-breaker safety layer for
PropChain contracts. It consolidates previously ad-hoc pause governance into
a single trait-based façade, adds an off-chain guardian daemon that watches
oracle drift and chain health, wires automatic rate-limiter tripping into
that health signal, and hardens DEX liquidity queries against
division-by-zero on fresh/empty pools.

Changes

Circuit-breaker façade (propchain-traits)

• Introduced propchain-traits::circuit_breaker::{CircuitBreaker, PauseReason}
  as a single uniform trait, replacing the per-contract PauseFlags,
  set_pause_state, and c.is_paused patterns.
• Migrated existing contracts to implement the shared trait so pause
  behavior and monitoring are consistent across the codebase.

Guardian-bot daemon (scripts/)

• Added a Rust binary at scripts/guardian-bot.rs that subscribes to chain
  events, watches oracle price feeds, and pauses the bridge automatically on
  detected drift.
• Documented deployment (README) covering configuration, run modes, and
  operational expectations.

Automatic rate-limiter tripping

• The bridge rate limiter now watches account_block_request_count and
  failed_signatures_window_count and automatically trips into a paused
  state when thresholds are breached.
• Added a cooldown timer governing auto-recovery after a trip.

Zero-liquidity guard

• Replaced unchecked division in DEX utilisation view functions with a
  guarded divide that returns an explicit ZeroLiquidity error instead of
  panicking/aborting on fresh or empty pools.

Closes

• Closes Ship a reference guardian-bot implementation in scripts/ #822 — Ship a reference guardian-bot implementation in scripts/
• Closes Add per-contract circuit-breaker façade in propchain-traits #818 — Add per-contract circuit-breaker façade in propchain-traits
• Closes Trip the rate limiter automatically on health degradation #819 — Trip the rate limiter automatically on health degradation
• Closes Fix zero-liquidity division-by-zero in DEX liquidity queries #823 — Fix zero-liquidity division-by-zero in DEX liquidity queries

Testing

• Unit tests covering circuit-breaker trait state transitions across
  migrated contracts.
• Integration test simulating oracle drift to confirm guardian-bot triggers
  a pause.
• Unit tests for rate-limiter auto-trip/cooldown/recovery cycle.
• Unit tests covering empty-pool and zero-liquidity queries returning
  ZeroLiquidity rather than aborting.

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LGTM