Exploring minimum-enshrined IB20 + wrapper architecture (alternative to variant design)

[amiecorso]

Summary

Alternative to the merged variant-based design (#1). Replaces the
Default/Stablecoin/Security variant interfaces (with capability bits,
role splits, two-step transfers, factory) with a single minimum
precompile bound to an EVM wrapper contract at creation.

The architectural unit: precompile + wrapper

Each B-20 token is a pair:

• Precompile (IB20): balance state, raw transfer mechanics,
  policy hook, pause flag.
• EVM wrapper contract: all authorization, all role logic, all
  product-specific behavior.

The pair is created atomically; the binding is permanent (wrapper()
is immutable on the precompile). Privileged operations (mint,
burn, wrapperTransfer, setPaused, setTransferPolicyId)
revert with OnlyWrapper for any other caller.

Mental model: OZ _internal / public split, but enforced at the
protocol level rather than by Solidity inheritance.

What's in the precompile (17 external functions)

• ERC-20 surface (name, symbol, decimals, totalSupply,
  balanceOf, allowance, transfer, transferFrom, approve)
• State reads (wrapper, transferPolicyId, paused)
• Wrapper-only operations (mint, burn, wrapperTransfer,
  setPaused, setTransferPolicyId)

What lives in the wrapper

Everything that was in the variant interfaces:

• Roles (admin, issuer, pauser, minter, RBAC)
• Two-step admin / issuer rotation with delays
• Capability bits / preset variants
• EIP-2612 permit, ERC-3009, transfer memos
• Per-minter rate limiting, supply caps
• Currency identifier, contract URI, asset type
• Share ratio, corporate actions, brokerage redeem flows
• Dividend distribution

A future PR can add reference wrapper implementations
(MinimalWrapper.sol, StablecoinWrapper.sol,
SecurityTokenWrapper.sol) to make these concrete.

Open questions

Captured in DESIGN_NOTES.md:

1. Wrapper rotation path (default: no, use proxy pattern internally)
2. Initial supply at creation (factory concern)
3. Burn-during-pause behavior
4. `setTransferPolicyId` validation against the registry

Files

• Add `src/interfaces/IB20.sol` (214 lines)
• Keep `src/interfaces/IPolicyRegistry.sol` (unchanged)
• Delete `Capabilities.sol`, `IDefaultToken.sol`, `IStablecoin.sol`,
  `ISecurityToken.sol`, `ITokenFactory.sol`
• Rewrite `DESIGN_NOTES.md`

Status

Draft. Posting for team reaction before deciding whether this
direction supersedes #1 or stays as a parallel sketch.