Feat/1037 key discovery registry

[Bolajiomo99]

Summary

This PR introduces the key-discovery-registry contract, a new on-chain discovery index for creator keys. It tracks keys deployed through the creator-keys factory, records per-key trade volume over rolling windows, and exposes sorted discovery views ("trending" and "new keys").

Key features:

• Registration: register_key(caller, key_id, creator) records deployment metadata (key_id, creator, deploy_timestamp). Only the admin or an authorised registrar may call this. Re-registering the same key is rejected.
• Volume tracking: record_volume(caller, key_id, amount) appends a positive volume entry for a registered key. Entries older than 48 hours are pruned on write. Only the admin or an authorised recorder may call this.
• Discovery views:
  • get_trending(limit) returns top keys by 24h volume (descending), ties broken by earlier deployment; keys with zero 24h volume are omitted.
  • get_new_keys(since_timestamp, limit) returns keys deployed at/after since_timestamp, newest-first.
• Housekeeping & reads: prune(key_id) removes records older than 48h (callable by anyone). Read-only helpers: get_volume_24h, get_volume_entries, get_key_info, get_registered_count.
• Admin: initialise(admin) (one-time), set_registrar(registrar, allowed), set_recorder(recorder, allowed). All admin operations require auth.
• Events: key_reg(key_id) with KeyInfo payload; vol_rec(key_id) with VolumeEntry payload.

The crate is added to the workspace. Unit tests cover registration/auth, volume windows/pruning, trending ordering, and new-keys filtering.

closes #1037

Testing

• cargo fmt --all -- --check
• cargo clippy -p key-discovery-registry --all-targets -- -D warnings
• cargo test -p key-discovery-registry

Checklist

• Linked issue or backlog item (closes Implement on-chain key discovery registry for trending and new key indexing #1037)
• Added/updated unit tests in key-discovery-registry/src/test.rs covering success and failure paths (unauthorised, duplicate registration, non-positive amount, unregistered key, pruning, trending, new keys). No creator-keys behavior changed, so no new creator-keys tests required.
• Ran cargo fmt --all -- --check, cargo clippy --workspace --all-targets -- -D warnings, and cargo test --workspace (all workspace tests pass; unrelated snapshot artifacts modified during workspace test run were reverted to keep scope clean)
• Reviewed persistent storage changes against docs/storage-key-invariants.md. New contract uses instance/persistent storage under its own DataKey namespace (Admin, Initialised, Registrar, Recorder, KeyInfo, KeyIndex, Volume). No existing storage keys are modified.
• Confirmed event names, topic order, payloads: events publish (EVENT_NAME, key_id) as topic and the corresponding struct as payload (KeyInfo for key_reg, VolumeEntry for vol_rec). Order/meanings align with existing Soroban event conventions.
• Updated docs: N/A (new contract; no existing public interface modified). README/docs can be updated separately if needed.
• Scope: limited to the new key-discovery-registry contract, workspace member addition, and its tests. No unrelated formatting, generated artifacts, or unnecessary dependency changes.

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