[NT7S]

Test-only change (no src/ changes) that locks v2.4.2 behavior before a targeted flash-size refactor.

• ctrl_status: 261 exact-byte checks for the clock control setters, drive_strength, fanout, update_status, get_i2c_error, and the NACK/no-data read paths.
• plan_branches: 55 checks for multisynth67_calc, set_freq_manual rejects, and untaken set_freq/set_ms/set_vcxo branches.
• equiv_vs_v242: hard pass/fail equivalence against a pinned v2.4.2 (f885fca) copy, 1386 runs, with a negative control.
• Flash budget check for Uno (arduino:avr 1.8.6): min_const 10830 B, min_runtime 11558 B ceilings.

Verified locally on the box (g++ 14.2): run_v231_checks.sh OVERALL PASS (38 pass, 2 info), negative control PASS, flash budget ALL PASS. Two existing quirks are locked as-is, not fixed (multisynth67_calc a=256 narrowing; set_pll caching an out-of-range request).