Implement the staking Contract's Staking, Rewards, and Unbonding Entrypoints #201

Description:
Reconstruct the staking crate from a non-compiling shell into a working Soroban contract with stake, unstake/unbond, and reward-claim entrypoints, matching whatever interface the crate's existing tests and callers imply.

Problem Statement & Context:
The task under way notes that the two newer crates staking and streams "currently fail to compile outright." Restoring them to a compiling state is being handled separately, but a stub that compiles provides no protocol functionality. staking matters because price_oracle feeder staking/slashing (covered elsewhere) and LP incentivization both depend on an actual staking primitive; without it those paths have nowhere to lock and reward.

Scope & Acceptance Criteria:

• Entrypoints for stake, initiate-unbond (with any lock/cooldown the design requires), withdraw-after-cooldown, and claim rewards.
• Correct accounting of per-staker principal and accrued rewards; no cross-staker leakage.
• Admin surface to configure reward rate / reward source, gated via common::require_admin_or_multisig if common is adopted here.
• Out of scope: any off-chain reward-funding tooling and frontend integration.

Implementation Guidelines:

1. Key Files/Modules: staking/src/lib.rs, staking/src/types.rs/storage.rs (create if missing), common/ helpers, staking test module.
2. Design/Architecture: typed persistent storage keyed by staker address for positions, plus a global reward-accumulator (index) so reward math is O(1) per action; keep math in a dedicated module.
3. Edge Cases/Constraints: zero-amount stake/unstake, unstake before cooldown elapses, staking while a prior unbond is pending, reward-accumulator overflow, and storage TTL extension on every write.
4. Testing: stake/unstake round-trip, two-staker reward-split correctness, cooldown enforcement, and unauthorized admin calls; assert no reward is claimable twice.

Definition of "Done":

• Code written, tested, and documented.
• All acceptance criteria met; crate compiles and its tests pass in CI.
• PR submitted with a clear description, passing CI, relevant test output.
• Code reviewed and approved.

Resources:

• README.md, Cargo.toml (workspace membership).

Complexity: High (8 points)