tri payout follow-up: re-bless the fetches census and keep GitHub's answers (the commits #8733 merged without) #8740

What

#8733 (see #8716) auto-merged at 18:11Z on its first commit, as soon as the four required checks passed. Three commits pushed after it are not on master:

1. The census re-bless. tri census pin --gate (cli-tri) now fails on master: the fetches census reads 49 files, not 48, because cli/tri/src/payout.rs names the GitHub API. It adds no fetch site (35 stays 35).
2. The GitHub answer cache. A cold tri payout run asks GitHub about every blamed commit that declares no agent (544 for epoch 20735). The glue should keep each answer that gh_read accepts in $TMPDIR/tri-payout-github/<sha> (GH_CACHE in specs/tri/network/payout.t27) and read it from there the next time. Measured: 204 s cold, 38 s warm, with the same digests.
3. The NOW note's numbers for the cache.

Done when

tri census pin --gate passes on master again, and a second tri payout run of the same epoch makes no GitHub call. payout.rs stays at 40 lines (net 0 hand-written foreign lines).

User Scenarios

• Given the fetches census is pinned at 48 files, when cli/tri/src/payout.rs names the GitHub API (adding one more fetch site), then tri census pin --gate fails because the census now reads 49 files.
• Given a cold tri payout run for an epoch makes 544 GitHub calls taking ~204 s, when the same epoch is run a second time, then no GitHub calls are made and the run completes in ~38 s using cached answers.
• Given the NOW note in the spec records cold/warm timings for the cache, when the cache is implemented, then the note's numbers reflect the measured 204 s cold and 38 s warm with identical digests.

Requirements

• FR-001: The fetches census MUST be re-blessed to 49 files so tri census pin --gate passes on master.
• FR-002: tri payout MUST write each accepted gh_read answer to $TMPDIR/tri-payout-github/<sha> (the GH_CACHE path from specs/tri/network/payout.t27).
• FR-003: tri payout MUST read cached answers from $TMPDIR/tri-payout-github/<sha> on subsequent runs, making zero GitHub calls for the same epoch.
• FR-004: cli/tri/src/payout.rs MUST remain at 40 lines (net 0 hand-written foreign lines).
• FR-005: The NOW note in specs/tri/network/payout.t27 MUST be updated with the measured cache timings (204 s cold, 38 s warm).

Boundary

• specs/tri/network/payout.t27
• cli/tri/src/payout.rs
• tools/census/fetches.txt
• specs/tools/tri/census.t27
• specs/tri/mutate/census.t27
• cli/tri/src/census.rs
• gen/rust/tri/network/payout.rs
• gen/rust/tri/mutate/census.rs