Fix C++20 compatibility after libcanon submodule update

[Copilot]

The libcanon submodule update to C++20 changed canonical form ordering due to std::unordered_map iteration differences between C++14 and C++20. This broke tests expecting specific index orderings (e.g., m[i,j] vs m[j,i]), though the forms remain mathematically equivalent.

Changes

Build configuration

• Updated CMakeLists.txt to require C++20 standard

Core logic (drudge/fock.py)

• Modified write_in_qp to accept equivalent canonical forms for matrix elements with identical creation/annihilation orders
• Replaced strict equality assertion with first-form-wins approach when multiple equivalent forms arise

Test expectations

• Updated canonical form expectations in term_test.py, free_algebra_test.py, genmb_test.py, parthole_test.py
• Replaced exact equality checks with difference-based comparisons where canonical form ordering is non-deterministic
• Changed structural validation to check algebraic properties (e.g., spin indices) rather than specific variable orderings

Example

Before C++20, canonicalization consistently produced:

# Input: sum over i,j of x[i,j] * v[i] * v[j]
# Output: sum_{i,j} x[i,j] * v[i] * v[j]

After C++20, may produce:

# Input: sum over i,j of x[i,j] * v[i] * v[j]  
# Output: sum_{j,i} x[j,i] * v[j] * v[i]  # equivalent but reordered

Both forms are mathematically identical due to summation commutativity and symmetry properties.

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