feat(contract): flash-loan prevention via sequence ID and price snapshot checks

[success-OG]

closes #267

[Contract] Flash-Loan Prevention via Sequence ID Checks

Summary

Implements an on-chain price snapshot guard that detects and blocks flash-loan-driven
price manipulation within the same ledger close, protecting the Chen Pilot vault.

What's included

• New flash_loan_guard contract added to the workspace
• initialize / update_config — configures oracle, guarded asset, deviation
  threshold (bps), and minimum ledger gap between snapshots
• record_snapshot — pulls the current oracle price and stores it with the current
  ledger sequence number. Enforces min_ledger_gap so an attacker cannot reset the
  baseline and exploit in the same ledger.
• assert_price_safe — the guard gate called before any price-sensitive vault op:
  1. Panics if no snapshot exists (cold-start protection)
  2. Panics if the snapshot was taken in the same ledger as the current call
     (same-block manipulation detection — the core flash-loan defense)
  3. Fetches the live oracle price and computes deviation from the snapshot
  4. Panics if deviation_bps > max_intra_ledger_deviation_bps
  5. Emits FlashBlk event on block, PriceSafe event on pass
• get_snapshot / get_config — read-only views
• 9 unit tests: happy path, no snapshot blocks op, same-ledger snapshot blocked,
  price spike blocked, small move allowed, min ledger gap enforced, snapshot update
  after gap, price drop blocked, double-init blocked

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