feat(onchain): add get_distributor_list accessor for indexers (#232)

[Moonwalker-rgb]

Summary

Closes #232

Adds a public view function get_distributor_list to the AidEscrow contract that returns a sorted Vec<Address> of currently registered distributor addresses.

Problem

Distributor list is stored in Map<Address, bool> but there is no view function to retrieve it. Off-chain indexers and dashboards must maintain a parallel list, which can grow stale and reduce audit accuracy.

Solution

• New view function get_distributor_list(env) -> Vec<Address> reads from KEY_DISTRIBUTORS storage and returns keys sorted for deterministic ordering
• No auth required (read-only view function, matching existing patterns like get_admin, get_config)

Changes

File	Change
src/lib.rs	+16 lines — added get_distributor_list function after remove_distributor
tests/aid_escrow_tests.rs	+95 lines — 6 test cases in new get_distributor_list module
README.md	+1 line — documented in Admin & Config table
docs/onchain/api.md	New file — high-level API reference

Test Coverage

• ✅ Empty after init with no add_distributor calls
• ✅ Returns single distributor after add
• ✅ Empty after add then remove
• ✅ Sorted list after 3 adds (verifies ordering and completeness)
• ✅ Reflects removal correctly
• ✅ Duplicate add is idempotent (Map key dedup)

Acceptance Criteria (from #232)

• ✅ Returns empty Vec after init with no add_distributor calls
• ✅ Returns sorted list after 3 add_distributor calls