bip360: depth-zero script trees should be anyone-can-spend

A merged pull request to the BIPs repository (opened 2026-06-13, merged 2026-06-19 as BIP-360 version 0.12.0) changing P2MR's validation rules: a script tree of depth zero — one containing only a single leaf script — now validates automatically, making such outputs spendable by anyone. Since funds sent there could be taken by any miner, the rule discourages single-leaf P2MR outputs entirely.

The motivation comes from the incentive critique of P2MR: a depth-zero tree is 32 bytes cheaper to spend than the same script inside Taproot, which could perversely encourage script protocols to drop their cooperative spending path or their post-quantum fallback leaf to save witness bytes. By making every P2MR user pay for at least a depth-one tree — two spending paths — the change removes that saving, encourages including a post-quantum leaf alongside the everyday Schnorr leaf, and makes P2MR spends look more alike on-chain.

The "anyone-can-spend" form (rather than an outright ban) was suggested in review by BIP-360 co-author Ethan Heilman so the pattern stays reusable by a future soft fork — for example, for post-quantum key-tweaking schemes not yet mature enough to specify.