test: add clock-rollover regression tests at u64 boundary

[senmalong]

Summary

Implements the regression test suite requested in issue #1585. Adds quicklendx-contracts/src/test_clock_rollover.rs and wires it into lib.rs with #[cfg(test)] (always-on, no feature flag required).

What is covered

Area	Happy-path	Sad-path
Invoice::is_overdue	not overdue at exact u64::MAX; safe after rollover to 0	overdue when due_date is small and clock is near MAX
Invoice::grace_deadline	saturates at u64::MAX from near-MAX due_date	—
Invoice::grace_deadline	saturates when due_date == u64::MAX and any grace period > 0	—
Bid::default_expiration	saturates at u64::MAX	—
Bid::is_expired	not expired at exact u64::MAX boundary	expired for small expiry near MAX
invoice creation (contract)	accepted at u64::MAX - 1 ledger / u64::MAX due_date	rejected when due_date == now == u64::MAX
invoice state (contract)	overdue query safe after simulated rollover	—
bid placement (contract)	expiration saturated when placed near u64::MAX	—
cleanup (contract)	no bid removed at exact saturated boundary	—

18 tests total — 8 pure-logic (no contract round-trip) and 10 integration (full contract calls).

Design decisions

• All deterministic: timestamps set via env.ledger().set_timestamp() and env.as_contract(). No Date::now() or random values.
• Test names are assertive: e.g. invoice_not_overdue_when_due_date_equals_u64_max.
• Always-on: the module is declared with plain #[cfg(test)], not behind legacy-tests or fuzz-tests, so it runs on every CI matrix entry.

Pre-existing failures

The codebase already has 22 compilation errors in unrelated files (verification.rs, lib.rs, test_invoice_search_ranking.rs, types.rs). These are not introduced by this PR and block cargo test for all tests on the current branch; they will need to be fixed separately.

Testing

# Run just the new tests (once pre-existing compile errors are resolved)
cargo test -p quicklendx-contracts test_clock_rollover

# Full suite
cargo test -p quicklendx-contracts

# Lint
cargo clippy --workspace --all-targets -- -D warnings

# WASM build verification
cargo build --target wasm32-unknown-unknown --release

Closes #1585

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