feat: add burn toll extension

[MyTH-zyxeon]

Implements #18.

Summary

• Adds contracts/extensions/BurnToll.sol, an optional mint/redeem toll hook with 0.5% default tolls, configurable burn token, floor-pool adapter, and minimum-depth gating.
• Wires Minter to route mint tolls and redeem tolls when the extension is enabled, while preserving current behavior when disabled.
• Adds Foundry coverage for toll math, low-liquidity skip behavior, governance reconfiguration, zero-toll config, and redeem payout accounting.
• Documents deployment wiring and rollback steps in docs/burn-toll.md.

Validation

• FOUNDRY_LIBS=["node_modules","/tmp/codex-forge-libs"] FOUNDRY_TEST=forge-test forge test -vv
• npx hardhat test
• forge fmt --check contracts/extensions/BurnToll.sol forge-test/BurnToll.t.sol
• git diff --check

[abhicris]

Welcome to kcolbchain, @MyTH-zyxeon — glad you're here. 🌱

Here's what happens from this PR:

1. Our automated review looks for obvious issues (tests, secrets, size) within a couple of hours.
2. If it's clean and CI passes, we merge without back-and-forth.
3. If we need changes, we'll leave a specific comment — not a generic nit. Push another commit and we re-review.

While you wait:

• Run the repo's tests locally (see the repo README.md).
• Keep the PR scoped to one concern — bigger PRs land slower.
• Don't commit tokens or .env contents.

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Thanks for writing the code. We're building this to last.

[abhicris]

🤖 Audit verdict: safe

New toll extension feature with proper access control, validated math, comprehensive tests, and no malicious code or credential leakage detected.

Audited by the kcolbchain PR pipeline. See pipeline docs.