chore(faucet): token-bucket rate limiting via reliakit-ratelimit

[satyakwok]

What

Adopt reliakit-ratelimit (one of the reliakit resilience crates) in the
testnet faucet, replacing two hand-rolled limiters.

• Per-IP: was a sliding-window Vec<Instant> that grew per IP — the doc
  comment already called it a token bucket, so this aligns the code with its
  stated intent. Now RateLimiter::new(max_drips, 1, window/max_drips):
  max_drips per window with a burst allowance, O(1) memory per IP.
• Per-recipient cooldown: was a hand-rolled last-timestamp check. Now a
  1-token bucket refilling every cooldown.
• Precise Retry-After hints from RateLimiter::retry_after.

Why it's safe

• Non-consensus: faucet binary only, testnet. No chain code touched.
• TOCTOU preserved: the per-recipient cooldown previously needed a DashMap
  Entry guard to stop two parallel requests for the same address from both
  passing. The new code keeps that guard — entry(..).or_insert_with(..) holds
  the per-key shard lock across try_acquire_one, so refill+consume is atomic.
• Rollback = git revert.

Tests

4 unit tests on the bucket mapping (burst-then-block, zero-max clamp,
one-per-cooldown, retry_after remaining). sentrix-faucet green under
RUSTFLAGS="-D warnings".

No workspace version bump (avoids colliding with the concurrent #788 bump);
changelog note under [Unreleased].

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: f6854e16-021a-4af2-998d-02f100dfc4ee

📥 Commits

Reviewing files that changed from the base of the PR and between 08f9738 and e8ebc85.

⛔ Files ignored due to path filters (2)

• CHANGELOG.md is excluded by !**/CHANGELOG.md
• Cargo.lock is excluded by !**/*.lock, !**/*.lock

📒 Files selected for processing (2)

• bin/sentrix-faucet/Cargo.toml
• bin/sentrix-faucet/src/main.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch chore/faucet-reliakit-ratelimit

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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