add short guide on fee system (#101)

Author: tac0turtle

docs/guide/fee-systems.md

@@ -0,0 +1,89 @@
+# Fee Systems Guide: Base Fee Redirect, FeeVault, Native Minting
+
+## Overview
+
+This guide connects three related components that move or manage native token value:
+
+- Base fee redirect: redirects EIP-1559 base fees to a configured sink instead of burning them.
+- FeeVault: a contract that accumulates native tokens and can split and bridge them.
+- Native minting precompile: a privileged mint/burn interface for controlled supply changes.
+
+These components are independent but commonly deployed together. The base fee redirect is a value transfer, not minting. Native minting is explicit supply change and should remain tightly controlled.
+
+## Base Fee Redirect (execution layer)
+
+**Purpose**: Redirect the EIP-1559 base fee to a sink address (EOA or contract) instead of burning it.
+
+**Mechanics**:
+
+- The EVM handler credits `base_fee_per_gas * gas_used` to the configured sink.
+- The redirect only activates at or after `baseFeeRedirectActivationHeight`.
+- If no sink is configured, base fee burns proceed as standard Ethereum behavior.
+
+**Chainspec configuration** (inside `config.evolve`):
+
+```json
+"evolve": {
+  "baseFeeSink": "0x00000000000000000000000000000000000000fe",
+  "baseFeeRedirectActivationHeight": 0
+}
+```
+
+See `docs/adr/ADR-0001-base-fee-redirect.md` for implementation details.
+
+## FeeVault (contract level)
+
+**Purpose**: Accumulate native tokens and split them between a bridge destination and a secondary recipient.
+
+**Mechanics**:
+
+- Receives base fees when `baseFeeSink` is set to the FeeVault address.
+- Anyone can trigger `sendToCelestia` (or equivalent) once the minimum threshold is met.
+- Splits balance by `bridgeShareBps`, sends the bridge share to `HypNativeMinter`, and transfers the remainder to `otherRecipient`.
+
+**Why it pairs with base fee redirect**: the redirect funnels base fees into the FeeVault automatically, turning burned fees into recoverable value for treasury or bridging.
+
+See `docs/contracts/fee_vault.md` for parameters and deployment details.
+
+## Native Token Minting Precompile
+
+**Purpose**: Provide a privileged, auditable way to mint or burn the native token.
+
+**Mechanics**:
+
+- Precompile address: `0x000000000000000000000000000000000000f100`.
+- `mintAdmin` manages the allowlist; both admin and allowlisted accounts can call `mint` and `burn`.
+- Activation is gated by `mintPrecompileActivationHeight`.
+- Setting `mintAdmin` to the zero address disables the precompile.
+
+**Chainspec configuration** (inside `config.evolve`):
+
+```json
+"evolve": {
+  "mintAdmin": "0x000000000000000000000000000000000000Ad00",
+  "mintPrecompileActivationHeight": 0
+}
+```
+
+See `docs/adr/ADR-0002-native-minting-precompile.md` for the full interface and security notes.
+
+## How They Fit Together
+
+1. **Base fee redirect** credits base fees to a sink address instead of burning them.
+2. **FeeVault** can be that sink, so base fees accumulate in a contract with deterministic split logic.
+3. **Native minting** is separate and optional; it is used for controlled supply changes (bootstrapping liquidity, treasury operations), not for redirecting fees.
+
+In other words, base fee redirect and FeeVault are about re-routing existing value, while native minting explicitly changes total supply. Keep those responsibilities separate and limit minting access to minimize systemic risk.
+
+## Suggested Deployment Pattern
+
+- Set `baseFeeSink` to the FeeVault address.
+- Use `AdminProxy` as the `mintAdmin` and FeeVault owner if you need a safe, upgradeable admin.
+- Activate both features at a planned height for existing networks.
+
+References:
+
+- `docs/contracts/admin_proxy.md`
+- `docs/contracts/fee_vault.md`
+- `docs/adr/ADR-0001-base-fee-redirect.md`
+- `docs/adr/ADR-0002-native-minting-precompile.md`