Support one-way, permanent conversion of a Principal Tree EMT to an EML

The Principal Tree is a mutable Epoch Merkle Tree (EMT). A mode should exist to convert a principal's EMT to an Epoch Merkle Log (EML) — one way and permanent: once converted, the tree becomes append-only and can never return to mutable mode.

This is a supported-mode design goal (Zami), not remediation of any current defect. It is feature work with protocol implications (the conversion presumably needs a transaction type, and the resulting principal's mutation semantics change), so it belongs to a future feature campaign rather than the current foundation-hardening effort. Tracked here so it isn't lost.

Related context: the eml library already models mutable trees and append-only logs as distinct structures; conversion likely composes with its existing collapse/promotion semantics. Design should start from a conversation with the spec author about the intended semantics before any implementation.

Start by discussing the intended conversion semantics with the spec author, as requested in the issue. Then read the eml library's mutable-tree and append-only-log structures, including its collapse and promotion semantics. Done means an agreed design for the one-way conversion, its transaction type, and the resulting principal mutation rules.

Written by the indexing model from the issue text.