EVM direct Asgard ERC20 deposits: add safe auto-refund path for ERC20 inbound transfers

Context (Incident Follow-up)

This issue is created in reaction to the recent memoless / direct-Asgard deposit incident discussed in THORChain Discord (InstaSwap user case, Feb 25–27, 2026): https://discord.com/channels/838986635756044328/1476157453736476703

Related follow-up issues from the same incident:

• #2776 - Memo preflight: validate memoless asset eligibility
• #2773 - Quote API: enforce memoless eligibility

Problem

Direct ERC20 transfers to Asgard (not via router / not executable as inbound swap) are left stranded, requiring manual recovery/refund and causing poor UX.

Proposal

Implement guarded auto-refund handling for direct ERC20 deposits to active Asgard vaults:

• Detect direct token deposits that are not valid router-based inbound swaps.
• Auto-refund when all safety conditions hold:
  • asset is supported/pool asset
  • amount exceeds refund gas + penalty threshold
  • amount exceeds "not dust" threshold
• Emit clear event/telemetry for refunded vs non-refundable cases.
• Add feature gate (Mimir/config) if needed for rollout safety.

Acceptance Criteria

• Direct ETH.USDT to Asgard (non-router) is detected and refunded when economically valid.
• No interference with valid router-based flows.
• Non-refundable deposits are logged - observable for manual handling.