Self-hosting slice 2a: the Queen feeds the network itself - master heads to shards, only what changed, a corpus receipt beside the lab's (TRIOS_HOSTING_FEED, off) #1867

Part of #1756 (self-hosting); release plan #1763.

Why

The network does not feed itself yet: every shard job is submitted by hand with the owner's key. The t27b labs, about $269 per 30 days, run every spec, both halves, for every master head. Two owner hosts with one slot each cannot do that, and they need not: a spec whose use closure and toolchain did not change keeps the row it had.

The rule

gHashTag/t27 specs/hosting/feed.t27 (gHashTag/t27#8973, merged in gHashTag/t27#8974) decides:

• which master heads are fed;
• which specs need shards: per spec and half, new, another toolchain, voided, changed closure, base half not a vote, or the 1-in-128 audit sample;
• how the corpus receipt is assembled, with the provenance of every half;
• when the receipt is complete;
• how it is compared with the lab's receipt.

This lane

• The card. Vendor feed.t27 and feed.wasm with their PIN lines, plus the t27c gen-ts constants.
• The feed, behind a new flag TRIOS_HOSTING_FEED (off by default; it also needs TRIOS_HOSTING=on):
  • it polls t27 master through the GitHub API within its budget;
  • per fed head, it reads one recursive tree and the spec files it has not yet seen (raw.githubusercontent.com);
  • it computes the shard set from the card;
  • it submits shard jobs through the Queen's own in-process owner path, never a host key;
  • it assembles the corpus receipt.
• Public reads, no secrets:
  • GET /hosting/corpus: the feeds;
  • GET /hosting/corpus/:sha: the receipt, with each half's provenance;
  • GET /hosting/corpus/:sha/lab: the comparison with the t27b lab's run of the same sha.
• Tests:
  • seeded, on a VirtualClock, with an in-memory GitHub fake;
  • parity at t27 cf42584c9 against the lab's rows: the fed specs from real host outputs, and the carried rows.
• A capacity estimate from real data: whether two host slots keep up with master, and what it takes to switch the labs off.

Not in this lane

• No merge into queen, and no Railway variable. The coordinator ships it and switches on TRIOS_HOSTING_FEED with a measured rehearsal.

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