Split multi-term reductions in hoist_invariants

[alexreinking]

Adjusts hoist_invariants (below in the stack) to return a FuncVec, where each entry is a separate handle to a parallel subterm, e.g. f() += a * g(r) + b * h(r) returns two intermediates (one for g, one for h), and replaces f with f() += a * f_intm0() + b * f_intm1() (i.e. scales are still hoisted).

Checklist

• Tests added or updated (not required for docs, CI config, or typo fixes)
• Documentation updated (if public API changed)
• Python bindings updated (if public API changed)
• Benchmarks are included here if the change is intended to affect performance.
• Commits include AI attribution where applicable (see Code of Conduct)

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[alexreinking]

I don't bother to bind FuncVec in Python because std::vector gets turned into a list, which supports destructuring natively in the language, like:

(f_intm,) = f.hoist_invariants()

[codecov]

Codecov Report

❌ Patch coverage is 90.16393% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.19%. Comparing base (8ed11d4) to head (e291904).

Files with missing lines	Patch %	Lines
src/Func.cpp	90.16%	0 Missing and 6 partials ⚠️

Additional details and impacted files

@@                    Coverage Diff                    @@
##           alexreinking/funcvecs    #9388      +/-   ##
=========================================================
+ Coverage                  70.06%   70.19%   +0.13%     
=========================================================
  Files                        261      261              
  Lines                      79483    79510      +27     
  Branches                   19382    19391       +9     
=========================================================
+ Hits                       55693    55816     +123     
+ Misses                     17911    17899      -12     
+ Partials                    5879     5795      -84     

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