docs: fix stale Base chain references (Arbitrum)

[koko1123]

Documentation-only: the README was stale against the code itself, which already targets Arbitrum (new_arbitrum, new_arbitrum_sepolia, ARBITRUM_CHAIN_ID = 42161, ARBITRUM_SEPOLIA_CHAIN_ID = 421614, ARBITRUM_USDC, ARBITRUM_SEPOLIA_USDC in src/client/mod.rs). No code changes.

Corrections

• "protocol on Base L2" -> "protocol on Arbitrum"
• Quickstart: "a Base Sepolia RPC" / "Fund your wallet on Base Sepolia" -> Arbitrum Sepolia
• Testnet USDC mint example: 0xC1a5D4E9... on sepolia.base.org -> 0xBEF280BefeE2Cb28c20D1E4Cc1da999B4DA0f1fD (PerpCity test USDC, not Circle's) on https://sepolia-rollup.arbitrum.io/rpc
• Client setup: shared_endpoint("https://sepolia.base.org") -> https://sepolia-rollup.arbitrum.io/rpc; chain id 84532 -> 421614
• HFT transport example: base.g.alchemy.com / base-rpc.publicnode.com -> arb-mainnet.g.alchemy.com / arbitrum-one-rpc.publicnode.com
• Env var table: default RPC https://sepolia.base.org -> https://sepolia-rollup.arbitrum.io/rpc (matches the actual defaults in examples/)
• Deployments section: hardcoded Base USDC -> ARBITRUM_SEPOLIA_USDC; "For Base mainnet, use PerpClient::new_base_mainnet() (chain 8453, USDC 0x8335...2913)" -> "For Arbitrum One, use PerpClient::new_arbitrum() (chain 42161) with ARBITRUM_USDC (0xaf88d065e77c8cC2239327C5EDb3A432268e5831)" — new_base_mainnet no longer exists

CHANGELOG.md untouched (historical record).

Follow-ups (code comments still mention Base, intentionally not touched in this docs-only PR)

• src/client/mod.rs:277: "every 1-2 seconds on Base L2"
• src/client/transactions.rs:31: "~2s on Base"
• src/hft/gas.rs:175,271: "2 Base L2 blocks", "typical Base L2"
• src/hft/state_cache.rs:3: "rate of change on Base L2"
• examples/hft_bot.rs:306: "~1s on Base L2"

Summary by CodeRabbit

• Documentation
  • Updated the README to reflect Arbitrum setup and configuration instead of Base.
  • Revised quickstart and deployment examples, including RPC endpoints, chain IDs, testnet funding, and USDC addresses.
  • Updated mainnet guidance and client initialization examples to match Arbitrum network settings.

[coderabbitai]

📝 Walkthrough

Walkthrough

README.md is updated to replace all Base network references with Arbitrum equivalents, including RPC endpoints, chain IDs, USDC contract addresses, and client initialization examples across the overview, quickstart, client setup, transport configuration, and deployment documentation sections.

Changes

README Arbitrum Migration

Layer / File(s)	Summary
Overview and quickstart updates README.md	SDK description now references Arbitrum; quickstart prerequisites and USDC funding cast send example use Arbitrum Sepolia RPC and contract address.
Client setup and transport configuration README.md	Client initialization uses Arbitrum Sepolia chain ID and RPC; HFT transport example uses Arbitrum mainnet Alchemy endpoint.
Deployments and environment configuration README.md	Default RPC_URL and Deployments snippet updated to Arbitrum Sepolia values; mainnet guidance now uses PerpClient::new_arbitrum() and ARBITRUM_USDC.

Estimated code review effort: 1 (Trivial) | ~5 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the docs-only update from Base to Arbitrum chain references.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch koko/readme-arbitrum-fix

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@README.md`:
- Around line 195-216: The README deployment example is using outdated
`Deployments` fields that do not match the public API. Update the example to
align with `Deployments` in `src/types.rs` by showing only the actual fields
(`perp` and `usdc`), and keep the Arbitrum Sepolia/Mainnet guidance consistent
with `PerpClient::new` and `PerpClient::new_arbitrum()` so readers can copy a
valid setup.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

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Run ID: 2771be27-c0f0-4097-a70f-1ee539a01bf9

📥 Commits

Reviewing files that changed from the base of the PR and between b1741a4 and ccee3ca.

📒 Files selected for processing (1)

• README.md