harden: add magic validation and buffer bounds to parseEngine/parseConfig

[0x-SquidSol]

Summary

• parseEngine and parseConfig can be called directly from adl.ts without going through parseHeader (which validates PERCOLAT magic)
• A malicious RPC returning arbitrary bytes matching a known slab size would be silently parsed into garbage engine state (vault balances, PnL, oracle data) or config (collateral mint, oracle authority)
• parseEngine also had zero buffer bounds checking — truncated RPC data would produce either garbage BigInts (silent wrong values) or cryptic DataView RangeError with no context

Additions:

• PERCOLAT magic check at the top of both parseEngine and parseConfig — one readU64LE comparison, zero overhead for valid slabs
• Buffer bounds check in parseEngine: data.length < layout.accountsOff ensures the buffer covers the entire engine section before any field reads

Test plan

• No new test failures (729 passed, same 16 pre-existing)
• All test mock slabs include PERCOLAT magic at offset 0 — zero false positives
• All test buffers are full-tier-sized — bounds check never trips on valid data

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: febb593e-9455-4024-b798-ed33a3ef99db

📥 Commits

Reviewing files that changed from the base of the PR and between 2f80d79 and 9764e1b.

📒 Files selected for processing (1)

• src/solana/slab.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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