Byte-verify ownership transfers and subasset reissuances; guard the borrow

[droplister]

Stacked on #216. Closes two of the three deliberate byte-equality gaps, and tightens a borrow shipped in #215.

Ownership transfers now byte-verify

The decline was over-conservative. A transfer's message is byte-for-byte a reissuance — core carries the new owner only in an output paying transfer_destination (issuance.py returns (source, [(destination, None)], data)). A new compose-oracle case proves it against a live node: core composes identical data with and without the transfer. So the message packs, equality covers every field, and the output policy — which already picks up transfer_destination via addressesNamedIn and requires every output to be explained — is what pins the ownership output. Same division of labor dispense relies on.

Subasset reissuances now byte-verify, under a borrowed id

Updating a subasset's description composes the standard layout under the ledger-resolved numeric id, which the request cannot determine. It's now borrowed from the composed message, like the initial issuance's random draw. The honest accounting: the field fallback is equally blind to a substituted id (it compares the longname the user typed, which the response echoes faithfully), so borrowing makes nothing worse and buys byte equality on the description, quantity, flags and MIME type.

The borrow refuses irreversible operations — on both subasset paths

A substituted id can land the operation on a different numeric asset the user owns (matching divisibility required; the range guard limits targets to numeric assets). For issue-and-describe the wrong outcome is recoverable — destroy the supply, rewrite the description. For lock, reset, and transfer it is permanent against the wrong asset, so the borrow refuses those and they stay on the field fallback: no blinder, but byte equality does not sign off on them.

This retroactively tightens #215's initial-issuance borrow, whose safety argument under-weighted the lock case. The full analysis is an addendum in the (deliberately uncommitted) audit log; the vector's real fix — an independent ledger view or local composition — is folded into the local-compose decision.

Verification

• tsc --noEmit clean; all 681 counterparty tests pass with both oracles live against api.counterparty.io:4000, including the new transfer oracle case.
• Fixtures regenerated with core's exact arithmetic for the unlocked subasset shape.
• E2E locally, one file at a time: issuance/index.spec.ts (10/10), transfer-ownership.spec.ts (3/3), update-description.spec.ts (3/3).

https://claude.ai/code/session_01CcjnCrgosSeshymXLBxdGj