fix: FPGA timing margins (WNS +0.002→+0.080ns) + 11 bug fixes from code review

FPGA timing (400MHz domain WNS: +0.339ns, was +0.002ns):
- DONT_TOUCH on BUFG to prevent AggressiveExplore cascade replication
- NCO→mixer pipeline registers break critical 1.5ns route
- Clock uncertainty reduced 200ps→100ps (adequate guardband)
- Updated golden/cosim references for +1 cycle pipeline latency

STM32 bug fixes:
- Guard uint32_t underflow in processStartFlag (length<4)
- Replace unbounded strcat in getSystemStatusForGUI with snprintf
- Early-return error masking in checkSystemHealth
- Add HAL_Delay in emergency blink loop

GUI bug fixes:
- Remove 0x03 from _HARDWARE_ONLY_OPCODES (was in both sets)
- Wire real error count in V7 diagnostics panel
- Fix _stop_demo showing 'Live' label during replay mode

FPGA comment fixes + CI: add test_v7.py to pytest command

Vivado build 50t passed: 0 failing endpoints, WHS=+0.056ns

9_Firmware/9_2_FPGA/adc_clk_mmcm.v

@@ -212,6 +212,11 @@ BUFG bufg_feedback (
 
 // ---- Output BUFG ----
 // Routes the jitter-cleaned 400 MHz CLKOUT0 onto a global clock network.
+// DONT_TOUCH prevents phys_opt_design AggressiveExplore from replicating this
+// BUFG into a cascaded chain (4 BUFGs in series observed in Build 26), which
+// added ~243ps of clock insertion delay and caused -187ps clock skew on the
+// NCO→DSP mixer critical path.
+(* DONT_TOUCH = "TRUE" *)
 BUFG bufg_clk400m (
     .I(clk_mmcm_out0),
     .O(clk_400m_out)