spec(tri/pr): preflight.t27 drops 7 tokens at parse -- use path through the keyword test #10233

Goal

Make specs/tri/pr/preflight.t27 pass the suite's parse-no-discard phase. Since #10220 the parser drops 7 tokens of the line use tri::test::report::opens_with;: the use path stops at the keyword test, so ::report::opens_with; is discarded. The Corpus Ratchet check is red on master (fdb6b28) and on every PR merged with it, for example #10185.

Boundary

• specs/tri/pr/preflight.t27: the use line goes, and a private opens_with built on the already imported span_is takes its place. The four call sites are unchanged.
• .trinity/seals/pr_TriPrPreflight.json and gen/rust/tri/pr/preflight.rs are regenerated by t27c.
• docs/reports/t27b_expectations.json: three rows, pass, for specs/ci/suite_ledger_steward.t27, specs/jobs/suite_ledger_steward.t27 and specs/tri/pr/preflight.t27. t27b-native-ratchet is red on master with UNLISTED 3 for exactly these (same shape as ledger(t27b): list spec_graph_derive, spec_graph_outline and t27mcp/find as pass (Closes #10212) #10214).
• The parser itself is not changed here.

Scenarios

• Given the change, When t27c parse-complete --show specs/tri/pr/preflight.t27 runs on the Railway lab, Then it reports nothing discarded.
• Given the change, When t27c test-report runs on the spec, Then 19 of 19 pass with 0 vacuous, the same as before the change.
• Given the new seal, When tools/check_seal_coverage.py runs, Then it exits 0.

Requirements

• FR-001: The spec MUST NOT import through a path segment that is a keyword.
• FR-002: opens_with MUST keep its meaning: false when at > end or lit does not fit, else span_is on text[at..at+lit.len].
• FR-003: The seal and the generated Rust MUST come from t27c, not be edited by hand.

Success Criteria

• On the lab, at master fdb6b28 plus this change: nothing discarded, 19/19 tests pass with 0 vacuous, and the seal check reports OK with 2392 of 2514 seals holding.

Close criteria

• The PR merges with its required checks green, and Corpus Ratchet no longer lists preflight.t27 under parse-no-discard.