feat: page invoice search results

[maxw06]

Fixes #1726.

Summary

• Add InvoiceSearch::search_invoices_paged(env, query, offset, limit) with a documented MAX_SEARCH_RESULTS cap and absurd-offset rejection.
• Keep search_invoices as a thin first-page compatibility wrapper.
• Add coverage for zero limit, above-cap limit clamping, offset-past-end, offset rejection, pagination ordering, and wrapper parity.

Validation

• git diff --check passes.
• cargo test -p quicklendx-contracts search_invoices_paged -- --nocapture currently fails before reaching these tests on inherited origin/main compile debt: duplicate bench module, duplicate compute_yield, missing idempotency / address_summary modules, and a non-exhaustive InvalidLedgerSequence match in errors.rs.
• cargo fmt is also blocked on current main by an unrelated parse error in quicklendx-contracts/src/test_bid_capacity_stress.rs.