test: honor stricter unit golden tolerances

[Streetblock]

Problem

Unit fixtures currently replace every requested tolerance with UNIT_TOLERANCE (8%). A test that explicitly requests 1% can therefore drift up to 8% while still passing. The failure message also reports the requested value instead of the effective one.

This was identified while reviewing the Code 128 mode-D FNC1 regression test.

Fix

• use min(requested, UNIT_TOLERANCE) for unit fixtures
• preserve the 8% default cap for callers requesting a looser tolerance
• honor explicitly stricter limits such as 1%
• report the actual effective tolerance on failure
• add a regression test for unit and non-unit selection

Verification

• cargo fmt --check
• cargo test --test e2e_golden -- --nocapture
• 104/104 tests pass on the standalone branch