Duplicated test helpers that no single-file fix can dedupe #783

Three helpers exist as byte-identical copies across test files. The /premerge simplify pass on #768 could not fix any of them, because its one-file-per-agent contract means no single-file edit can honestly dedupe a cross-file copy — two agents returned NO_CHANGE rather than half-do it, which is the right call and the reason these need their own change.

This is the "one contract, N uncompared copies" class (#370/#371). Distinct from #780, which lists patterns where one copy was widened and its twins left narrow; these are cases where the copies still agree and the risk is future drift.

1. The shell function-body extractor — 2 copies

g56_body (tests/goal.test.sh:4820) and rcx15_body (tests/review-pr-consolidate.test.sh:897) are identical down to their comment text and their inf && /^}/ { exit } terminator. tests/goal.test.sh already sources tests/_lib_assert_structural.sh, which has no body extractor.

The terminator is the risk, and it is demonstrated. It bounds the body at the first column-0 }, so a function containing a heredoc with a column-0 } truncates early. Measured: a _uberdev_goal_rebase_collision_chain fixture with such a heredoc extracts 4 of its 8 body lines, and the offender detector then returns zero hits over a body carrying two live unpinned sorts and one unpinned comm — G56.live would print PASS over nothing.

Harden one copy and the other keeps truncating. The >= 10 line-count floors already present in both files do red that case, so they backstop the move.

Fix: define it once in tests/_lib_assert_structural.sh, call from both. Three files.

2. The workflow run-block extractor — 3 copies

The awk program that carves a job's run: block out of .github/workflows/test.yml, including the hardcoded ten-space de-indent, exists in tests/ci-shard.test.sh and tests/ci-wiring.test.sh:651. #781 collapsed the two copies inside ci-shard.test.sh into one run_block_of; the ci-wiring.test.sh copy is untouched and out of that PR's scope. A fourth partial copy of the job-block half lives in tests/ci-job-fixtures.sh, whose own header names this defect class as the reason it exists as one shared file.

Drift has already started: a sibling extractor at tests/ci-shard.test.sh:554 carries a trailing-CR strip for the Windows checkout and the others do not. #781 deliberately did not propagate that strip, because widening what the extractor accepts is a behaviour change rather than a dedupe.

3. The tools: frontmatter predicate — 4 copies

#780 covers the two narrow ones. Adding the state after #781: tests/testers-agent-contract.test.sh and tests/model-routing.test.sh now agree, but only by construction — two byte-equivalent predicates in two files with nothing comparing them, which is the same shape #749 was made of. tests/cluster.test.sh:154,160 and tests/orchestrator-plan-flatten.test.sh:97,181 remain narrow.

Fix: one importable definition all four call, rather than four that happen to match.

Suggested shape for all three: hoist one definition per helper and give it a CONTRACT: marker, so the next divergence is visible rather than silent. The line-count floors already in place are what make the moves safe to verify.

Related: PR #768, PR #781, #780