Merge branch 'chat/simple-alu-formal-verif'

Add formal verification files for a simple ALU module.

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+# simple_alu Formal Verification
+
+Run with: `sby -f simple_alu.sby` from this directory.
+
+Reference: https://yosyshq.readthedocs.io/projects/sby/en/latest/reference.html
+
+## Mode
+
+Bounded Model Check (BMC, depth=100): proves assertions hold for the first 100 clock cycles.
+A counterexample waveform is generated on failure.
+
+Prove mode (k-induction): proves assertions hold for infinite time. Much stronger guarantee but harder to converge. Change `mode bmc` to `mode prove` in the `.sby` to use it.
+
+## Properties
+
+| Name              | Status      | Description                              |
+|-------------------|-------------|------------------------------------------|
+| `a_rst_check`     | implemented | y == 0 one cycle after reset asserts     |
+| `a_rst_overflow`  | planned     | overflow == 0 one cycle after reset      |
+| `a_add`           | planned     | y == a+b one cycle after sel=0           |
+| `a_sub`           | planned     | y == a-b one cycle after sel=1           |
+| `a_overflow_add`  | planned     | overflow correct for addition            |
+| `a_overflow_sub`  | planned     | overflow correct for subtraction         |
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