Fix verilog_reader importing NOR and XNOR assignments as constant zero

[vera-rubin]

verilog_reader inherits lorina's empty on_nor and on_xnor callbacks, so parsed output assignments such as assign y = ~(a | b); and assign y = ~(a ^ b); become constant zero.

This adds both callbacks with the same module and operand checks as on_nand. They negate create_or and create_xor with create_not, preserving operand complements without requiring direct NOR/XNOR methods from every network.

Tested against base 1c9c3af6a423f650f654ccec340dae9b93995844 in a GCC Debug build. The new MIG regression covers NOR, XNOR, a NAND control, and complemented operands. It fails four of five functional checks before the fix and passes afterward. The full run_tests suite passes before and after. The issue reproducer passes after the fix with assertions enabled and disabled.

A separate four-expression fixture also checks AIG, XAG, XMG, k-LUT, MuxIG, crossed k-LUT, and buffered crossed k-LUT. Each imports the expected truth tables. Release builds and the other CI toolchains haven't been checked locally.

Fixes #712

ChipLab ref: CLAB-2026-353

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.06%. Comparing base (1c9c3af) to head (81bdbf5).

Files with missing lines	Patch %	Lines
include/mockturtle/io/verilog_reader.hpp	66.66%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #713      +/-   ##
==========================================
- Coverage   84.10%   84.06%   -0.04%     
==========================================
  Files         191      191              
  Lines       29579    29606      +27     
==========================================
+ Hits        24876    24887      +11     
- Misses       4703     4719      +16     

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[myskyko]

Thank you for the PR. Why don't we use create_nor and create_xnor? Probably we don't need the function description comment for this file.

[vera-rubin]

Thanks! Done: it now uses create_nor/create_xnor when the network provides them and falls back for networks that don't (mig_network has no create_xnor, klut_network has neither). I also dropped the function comments.