t27b: lower try std.testing.* statements in tests (blocker ExprUnary(try) statement) #6354

Refs #6063

Problem

ExprUnary(try) statement is the top greedy blocker of the t27b corpus at master fefecd8: 5 reference-passing files (seed, bus, cognitive_loop, phi_timing, trinity_numeric_surface) are refused only because of it.

Goal

Lower try std.testing.expect / expectEqual / expectApproxEqAbs(...) as a statement in a test in cli/t27b/src/lower.rs so per-test verdicts equal the reference (t27c gen + zig test): a failed check fails that test alone, later tests still run, a negative tolerance panics (fails the test), NaN fails approxEqAbs.

Boundary

Where the reference's Zig does not compile the statement, refuse with a named blocker: try in an invariant (comptime block), in a reached fn (non-error return type), on a user fn, approxEqAbs on comptime floats or non-floats, other std.testing.*. try in a bench or unreached fn is never analyzed and lowers to a stub trap. Keep #6353's ignored-result rule.

Acceptance

• t27b corpus specs --blockers --reference <t27c>: mismatch 0, no regression, counted pass up.
• tests in cli/t27b/tests/source.rs whose verdicts are confirmed under t27c test-report.