T-rollup: rollup bot merges ready PRs into one commit (≤200 lines via gh, close constituents only after SHA on main) #5737

Refs #5510 (EPIC 20 merged/h), #5394. Ordered by aprender-57 (operator ruling 4); intake line from aprender-flow (T-rollup). Owner aprender-w5404.

Operator ruling (verbatim, relayed)

"Rollup bot checks the <=200-line rule itself via gh additions+deletions, not blocked on #5733. Bot may close a constituent only after the rollup commit is on main, with the SHA posted; never on red; ejected PRs stay open with a reason."

Loss

Green PRs queue up behind each other and every merge dirties the rest (#5394). A bot that rolls ready PRs into one merge removes the per-PR queue cost.

Change

A rollup bot (new crate aprender-rollup, work in flight on branch feat/rollup-bot): pure selection/split/outcome core first, then the gh I/O layer and a bin.

Falsifiers (test first)

• A candidate whose additions+deletions exceed 200: not selected (rule read from gh, not from PR-size check in PR CI: changed lines per PR, derived files excluded (p90 ≤200 gate) #5733).
• A rollup of more than 10 PRs: split.
• Overlapping files between candidates: not rolled together.
• A red candidate: ejected, stays open with a stated reason.
• A constituent is closed only when the rollup commit is on main and its SHA is posted; never otherwise.

Process

One ticket, owner w5404. The new crate touches Cargo.toml/Cargo.lock: a3 confirms. No PR until open ≤10 (operator freeze). Workflow or gate changes need "gate/workflow sign-off: YES".

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