feat: add rate-limited faucet for simulated token distribution

[obswrld]

closes #213

Summary

Adds a rate-limited faucet module to the counter contract, allowing new users to claim simulated tokens to start trading.

Changes

New file: counter/src/faucet.rs

• claim_faucet(env, user, asset) — mints a configured drip amount per user/asset with per-user cooldown enforcement
• set_faucet_config(env, caller, asset, drip_amount, cooldown_secs) — admin-only configuration of per-asset drip amount and cooldown
• get_faucet_config(env, asset) — query faucet configuration
• 16 tests covering cooldown boundaries, admin permissions, balance accumulation, independent per-user/per-asset cooldowns, and edge cases (zero cooldown, boundary second, config updates)

Modified files

• errors.rs — FaucetRateLimited (900), FaucetNotConfigured (901)
• events.rs — FaucetClaimed event emission
• lib.rs — Module registration and #[contractimpl] entry points (claim_faucet, set_faucet_config, get_faucet_config)
  Acceptance criteria
• First claim mints the configured amount and updates balance
• Second claim within cooldown rejects with FaucetRateLimited
• Claim after cooldown succeeds
• Admin can set drip amount and cooldown; non-admin cannot
• Tests cover cooldown boundaries via ledger timestamp manipulation