Auctions: Precise and safe time conversions (SystemTime→µs), correct U256 topic filtering, clearer errors

[gubatron]

This patch removes ad-hoc time conversions and establishes a single, correct, and auditable path for turning SystemTime into microseconds across the auctions client. It also ensures event filtering uses the right on-chain width (U256) and tightens error messages so unit mismatches are obvious.

What changed

• New helpers inside AuctionClient:
  • micros_from_system_time(SystemTime) -> u128
  • micros_u64_from_system_time(SystemTime) -> anyhow::Result<u64> (overflow-checked)
  • micros_u256_from_system_time(SystemTime) -> U256
• fetch_bids_for_deadline now filters with topic3(Self::micros_u256_from_system_time(deadline))
• submit_bid now converts the deadline via micros_u64_from_system_timereplacing the previous inline try_into()
• Error text corrected from "seconds" to "microseconds" to reflect actual units.

Why this is needed

• Correctness & type fidelity: The auction ABI uses microseconds; topics are 256-bit. Converting once and feeding the exact expected types eliminates subtle bugs (e.g., ms/us mismatches, accidental narrowing).
• Overflow safety: Far-future deadlines now fail fast with a precise error instead of silently truncating to u64
• Maintainability: Centralizing the conversions prevents drift and duplication. If our time representation changes, we update in one place. DRY principle.
• Operator clarity: Accurate error messages shorten incident triage when a caller provides an out-of-range timestamp.

Risk assessment

• Low. This is a client-side refactor preserving semantics for valid inputs. The only behavior change is improved validation (explicit overflow error) and more accurate error messages.
• Gas/chain impact: None. Contract code and calldata formats are unchanged.
• Backwards compatibility: Call sites passing reasonable deadlines continue to behave identically.

Notes for reviewers

• The helpers are intentionally fn (not const fn) to keep the surface minimal and focused on correctness rather than compile-time evaluation.
• Naming uses µs/micros consistently; this PR also scrubs the last misleading “seconds” mention in error text.
• If we later store deadlines differently (e.g., milliseconds or a newtype), swapping the internals of these helpers will propagate safely.

Checklist

• Centralized conversions
• Correct U256 topic filtering
• Overflow-checked u64 path
• Clear, unit-accurate error messages
• No public API breakage

If there’s any preference to move these helpers to a shared time utils module, I can follow up with a separate, mechanical PR.

Cheers!

[abresas]

LGTM