PR #93 added a 2-frame confirmation debounce so a single-sample GPIO
glitch cannot flip MCU outer-loop AGC state. The debounce is load-bearing
for the "prevents a single-sample glitch" guarantee in the PR body, but
no existing test enforces its structure — test_mcu_reads_dig6_before_agc_gate
only checks that HAL_GPIO_ReadPin(FPGA_DIG6, ...) and `outerAgc.enabled =`
appear somewhere in main.cpp, which a naive direct assignment would still
pass.
Add test_mcu_dig6_debounce_guards_enable_assignment to
TestTier1AgcCrossLayerInvariant, verifying four structural invariants of
the debounce:
1. Current DIG_6 sample captured in a local variable
2. Static previous-frame variable defaulting to false (matches FPGA
boot: host_agc_enable resets 0)
3. outerAgc.enabled assignment gated by `now == prev`
4. Previous-frame variable advanced each frame
Verified test fails on a naive patch that removes the guard and passes
on the current PR #93 implementation. Full cross-layer suite stays at
0 failures (36/36 pass locally).
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