feat(contracts): loyalty rewards contract for consecutive on-time renewals (#1045)

Add a new `loyalty_rewards` Soroban contract that accrues loyalty points
for subscribers who renew on time, with streak tracking and a redeem path
against fees.

## Contract mechanics

- **accrue(owner, sub_id, renewal_ledger)** — called by the authorised
  renewal caller after a successful on-time renewal. Awards
  `BASE_POINTS` (100) + `streak × STREAK_BONUS` (50), with the streak
  multiplier capped at `MAX_STREAK_BONUS_LEVEL` (20) to bound the max
  per-renewal award at 1 100 points.

- **miss(owner, sub_id)** — called when a renewal is missed or late.
  Resets the consecutive streak to 0; does not deduct points.

- **redeem(owner, amount)** — burns `amount` points from the owner's
  balance and returns the equivalent fee credit (1 point = 1 credit unit).
  Enforces a minimum redemption of 100 points.

## Storage layout

| Key | Storage tier | Description |
|-----|-------------|-------------|
| `ConfigKey::Admin` | Persistent | Contract administrator |
| `ConfigKey::RenewalCaller` | Persistent | Authorised renewal contract address |
| `ConfigKey::Paused` | Persistent | Global pause flag |
| `UserKey::Account(addr)` | Instance | Per-user `LoyaltyAccount` struct |

## Errors

| Code | Name | Trigger |
|------|------|---------|
| #1 | NotInitialized | Contract not yet initialised |
| #2 | AlreadyInitialized | `init` called twice |
| #3 | Unauthorized | Auth check failed |
| #4 | Paused | Contract is paused |
| #5 | RedeemTooSmall | `amount < MIN_REDEEM` |
| #6 | InsufficientPoints | Balance < requested amount |
| #7 | Overflow | Arithmetic overflow guard |

## Test coverage (35 tests)

- init: success + double-init guard
- accrue: base points, streak increment, bonus arithmetic, balance
  accumulation, lifetime points, last-renewal ledger recording
- accrue: streak bonus hard-capped at MAX_STREAK_BONUS_LEVEL
- miss: streak reset, no point deduction, zero-streak noop, restart
  after miss
- redeem: burns points + returns credit, full-balance drain,
  total_redeems counter, lifetime_points invariant
- redeem error paths: below minimum, insufficient balance, empty account
- pause/unpause: accrue / miss / redeem all reject when paused
- set_renewal_caller: admin can rotate the caller address
- multiple independent users: balances and streaks are isolated
- account / streak / balance queries
- uninitialized contract guard

Closes #1045

Author: wendypetersondev

contracts/Cargo.toml

@@ -11,6 +11,7 @@ members = [
   "contracts/payment-channel",
   "contracts/contract-upgrade",
   "contracts/allowance",
+  "contracts/loyalty_rewards",
 ]
 
 [workspace.dependencies]

contracts/contracts/loyalty_rewards/Cargo.toml

@@ -0,0 +1,14 @@
+[package]
+name = "loyalty_rewards"
+version = "0.0.1"
+edition = "2021"
+publish = false
+
+[lib]
+crate-type = ["cdylib"]
+
+[dependencies]
+soroban-sdk = { workspace = true }
+
+[dev-dependencies]
+soroban-sdk = { workspace = true, features = ["testutils"] }