Fix symbolic rounding with ForwardDiff 1.4.5

[ChrisRackauckas-Claude]

Ignore this PR until it has been reviewed by @ChrisRackauckas.

Summary

Restore symbolic floor and ceil differentiation after ForwardDiff 1.4.5 switched Dual rounding to Base's round(x, ::RoundingMode) interface.

The compatibility methods are confined to SymbolicsForwardDiffExt and only apply when the dual primal is Num. The regression test directly checks both symbolic discontinuities.

Regression boundary

Clean Symbolics master at 30e97f6b5:

• ForwardDiff 1.4.4: test/forwarddiff_symbolic_dual_ops.jl passes
• ForwardDiff 1.4.5: fails at ceil(Dual{...,Num}) because ForwardDiff calls round(::Num, RoundUp)

The behavior changed in ForwardDiff PR #829 / commit 569af35:
JuliaDiff/ForwardDiff.jl#829

ForwardDiff's new method is correct for its rounding interface. Symbolics owns the cross-package compatibility because Num supports symbolic floor and ceil without implementing every Base rounding mode.

Local verification

Julia 1.12.6, ForwardDiff 1.4.5:

• focused ForwardDiff extension test: passes, including the two new assertions
• full GROUP=Core Pkg.test(): 17,105 passed, 355 pre-existing broken, exit 0

Julia 1.10.11, ForwardDiff 1.4.5:

• focused ForwardDiff extension test: passes, including the two new assertions

Additional:

• Runic.jl 1.7.0 was run in-place on every changed line and made no changes
• git diff --check: passes

The clean-default failure was reproduced before this branch was created; no changes from the public fixpoint_sub PR are included here.

[ChrisRackauckas-Claude]

CI dependency note:

• The Spell Check failure is the clean-master shs failure fixed separately in Fix inherited discontinuity spell-check failures #1946.
• The benchmark job is expected to inherit the clean-master fork-comment HTTP 403 fixed separately in Use job summaries for fork benchmarks #1947.
• This PR remains limited to the ForwardDiff 1.4.5 symbolic-rounding regression; its test (Core, 1) result is the relevant package-CI check.

Merge/rebase order: #1946, #1947, then rebase this PR onto the resulting master.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.29%. Comparing base (1a5328d) to head (fbc41dc).
⚠️ Report is 1 commits behind head on master.
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Additional details and impacted files

@@             Coverage Diff             @@
##           master    #1948       +/-   ##
===========================================
+ Coverage   23.43%   80.29%   +56.86%     
===========================================
  Files          61       61               
  Lines        5995     6086       +91     
===========================================
+ Hits         1405     4887     +3482     
+ Misses       4590     1199     -3391     

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