BRC-143: Subtree data frame format

[jefflightweb]

This pull request:

Proposes a new standard by creating a new markdown file in the appropriate directory and requests discussion and assignment of a BRC number

Summary

BRC-143: Subtree Data Frame Format

Adds BRC-143, the push wire form of a subtree — an ordered list of transaction node hashes plus the merkle root that identifies it — for delivery and ingest over a byte stream. A subtree names its members by hash; a receiver reconstructs the subtree and verifies it against the carried root. All frame integers are fixed-width big-endian; the sole delimiter is the node count.

Motivation: in an announce/pull system a receiver fetches a subtree by its root hash (the root is the request key, carried out-of-band in the request path) and the bytes transferred are a bare list of 32-byte node hashes. A push delivery has no request path — the subtree arrives unsolicited — so the identifying root must travel in-band. BRC-143 is that frame: root + ordered node hashes, and nothing a receiver recomputes locally. Per-node fee/size, the aggregate totals, and the conflict set are omitted (fee/size come from the receiver's transaction-meta store; the rest aren't consumed on the ingest path) to avoid inflating an already large object.

Changes: new transactions/0143.md; index entries in README.md, SUMMARY.md, transactions/README.md. No changes to existing BRCs.