t27b-lab: a request whose commit the lab has not fetched yet waits instead of being dropped #8320

Part of the t27b overnight improvement loop (see the epic that links here).

Problem

corpus-receipt admit (specs/verified/corpus_receipt.t27, request_verdict / request_exit) drops a request at once when its sha is not a branch head on origin or on master's first-parent line as the lab last fetched it. On 2026-10-09 this dropped four lane requests (42f5946, 3d04e57, 8298f50, d0b2931) and the first request for #8028's squash commit c6237a7, which the lab had not fetched yet. A post-merge receipt is the only practical receipt while master moves every 10-20 minutes.

Change

A NOT_ON_ORIGIN request younger than a grace period (30 min) gets a new verdict WAIT_ORIGIN, whose exit code is 2. lab.py already keeps any request whose admit exit is neither 0 nor 1, so no hand-written line changes. After the grace it is dropped as before.

Acceptance

• Spec tests cover: young + not on origin -> WAIT_ORIGIN, exit 2; old + not on origin -> NOT_ON_ORIGIN, exit 1; on origin -> ADMIT, exit 0.
• gen-rust regenerated, seal re-saved, t27c builds.

User Scenarios

• Given a request whose commit SHA is not on origin/master as the lab last fetched it, and the commit is younger than 30 minutes, When corpus-receipt admit runs, Then the verdict is WAIT_ORIGIN with exit code 2.
• Given a request whose commit SHA is not on origin/master as the lab last fetched it, and the commit is older than 30 minutes, When corpus-receipt admit runs, Then the verdict is NOT_ON_ORIGIN with exit code 1.
• Given a request whose commit SHA is on origin/master as the lab last fetched it, When corpus-receipt admit runs, Then the verdict is ADMIT with exit code 0.

Requirements

FR-001: The request_verdict function in specs/verified/corpus_receipt.t27 MUST return WAIT_ORIGIN for a request that is NOT_ON_ORIGIN and whose commit age is less than 30 minutes.
FR-002: The request_exit function in specs/verified/corpus_receipt.t27 MUST return exit code 2 for WAIT_ORIGIN verdict.
FR-003: The request_verdict function MUST return NOT_ON_ORIGIN for a request that is NOT_ON_ORIGIN and whose commit age is 30 minutes or older.
FR-004: The request_exit function MUST return exit code 1 for NOT_ON_ORIGIN verdict.
FR-005: The request_verdict function MUST return ADMIT for a request whose commit is on origin/master.
FR-006: The request_exit function MUST return exit code 0 for ADMIT verdict.
FR-007: Spec tests in specs/verified/corpus_receipt.t27 MUST cover the three verdict cases (WAIT_ORIGIN, NOT_ON_ORIGIN, ADMIT) with their respective exit codes.
FR-008: After spec changes, gen-rust MUST regenerate successfully, the seal MUST be re-saved, and t27c MUST build without errors.

Boundary

• specs/verified/corpus_receipt.t27
• specs/queen/waits.t27
• specs/port/trios/crates/trios-git/src/commit.t27
• scripts/tri_loop/t27b.py