test(dispute): add timeline ordering and monotonicity property tests

[Agbasimere]

Description

Adds a doc-backed dispute timeline property-test harness and tightens the dispute state machine so timeline ordering, timestamp monotonicity, and terminal resolution are enforced consistently.

Closes #1210

What changed

• added docs/dispute-timeline-invariants.md as the executable source of truth for dispute timeline ordering
• added quicklendx-contracts/src/test_dispute_timeline_props.rs
• added doc-sync assertions so drift between the invariant doc and the test model fails the suite
• persisted an exact UnderReview timestamp for disputes so the timeline can be strictly monotonic
• enforced forward-only dispute transitions:
  • create only from None
  • update_dispute_evidence only while Disputed
  • put_dispute_under_review only from Disputed
  • resolve_dispute only from UnderReview
• exposed get_dispute_timeline through the contract surface
• documented the audit-trail interplay note that the user-facing timeline is not a replacement for the append-only audit trail

Invariants covered

• Opened -> UnderReview -> Resolved is the only visible timeline order
• no duplicate lifecycle entries can appear in the returned timeline
• accepted transitions produce strictly increasing timeline timestamps when ledger timestamps increase
• illegal orderings are rejected deterministically
• Resolved is terminal and cannot be re-reviewed, re-resolved, or have evidence rewritten

Testing

Validated locally with:

• cargo check --lib
• cargo check --tests

I also ran cargo test test_dispute_timeline_props --lib far enough to confirm the new tests themselves passed:

• test_dispute_timeline_props_doc_sync
• test_dispute_timeline_props_resolve_is_terminal
• test_dispute_timeline_props_randomized_sequences

The full command could not complete cleanly in this Windows GNU environment because linking/artifact generation eventually ran into local disk-space exhaustion in quicklendx-contracts/target.

Notes

• the randomized harness covers at least 20,000 generated action sequences
• unrelated local modifications in:
  • quicklendx-contracts/tests/backup_retention_validation.rs
  • quicklendx-contracts/tests/invoice_lifecycle_e2e.rs
    were left untouched and are not part of this commit

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