BRC-148: Multicast shard domain partitioning and BEEF object plane

[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-148: Multicast Shard Domain Partitioning and the BEEF Object Plane

Adds BRC-148, a backward-compatible partition of the BRC-129 16-bit shard-index space into independent object planes selected by the high nibble, and the allocation of the first new plane: a BEEF object plane (domain 0x1) carrying BRC-62/95/96 objects verbatim for overlay synchronisation. The plane shards by overlay topic — a topic name hashes to exactly one multicast group, so the group itself is the coarse topical filter — and delivery edges filter per subscriber on elected topics and BEEF encodings. A per-domain descriptor extension to the BRC-139 shard manifest lets each plane advertise and reshard its own shard_bits and generation independently. The transaction plane (domain 0x0) is byte-identical to BRC-129; a BRC-129-only participant interoperates unchanged.

Motivation: BRC-88 overlay propagation discovers every interested host and sends each a copy point-to-point over HTTPS — the step that does not scale as topics and hosts multiply. This plane gives it a multicast substrate: publish once, every subscribed host receives it, with NACK retransmission. Topic-hash addressing keeps the fabric free of per-topic state, so channel counts and retransmission bookkeeping stay bounded at millions of topics; the plane reserves an eight-fold widening runway (shard_bits up to 15) without renumbering.

Changes: new transactions/0148.md; index entries in README.md, SUMMARY.md, transactions/README.md. No changes to existing BRCs.

[jefflightweb]

Marking this draft while I revise the BEEF submission-record design. The multi-topic submission record (topicCount 1..15) fans out one ingress object into up to 15 multicast frames, which — on the open/anonymous ingress class — is an amplification vector with no cost coupling. The revision makes multi-topic fan-out an authenticated/metered capability and keeps the open path single-topic. Will mark ready for review once it lands.

[sirdeggen]

Merged as BRC-228 (renumbered from proposed BRC-148), commit e32374e on master.