[Do not merge] Replace Polynomial/poly! with symbolic expression system

[isPANN]

Summary

• Added src/expr.rs: symbolic expression system with AST, Pratt parser, evaluator, Display, and Serde (string round-tripping)
• Migrated all 30 reduction files from poly! macro to string expression literals (e.g., "3 * n ^ 2 + m")
• Migrated ReductionOverhead, export system, cost functions, and graph module to use Expr
• Deleted src/polynomial.rs and poly! macro (replaced entirely by Expr)

What Changed

The overhead DSL now supports arbitrary expressions beyond polynomials: exponentiation with non-integer bases (1.44 ^ n), built-in functions (log2, sqrt, min, max, floor, ceil), and full arithmetic. Expressions are parsed from string literals at construction time and serialize as strings in JSON.

Test Plan

• All 1468 lib tests pass
• All 47 doc tests pass
• Clippy clean (no warnings)
• Formatting clean
• Examples compile and run
• Reduction graph JSON regenerated successfully

[codecov]

Codecov Report

❌ Patch coverage is 84.80447% with 136 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.96%. Comparing base (f3d42d5) to head (b6681ba).

Files with missing lines	Patch %	Lines
src/expr.rs	81.37%	87 Missing ⚠️
src/rules/cost.rs	17.14%	29 Missing ⚠️
src/rules/graph.rs	68.18%	7 Missing ⚠️
src/rules/registry.rs	84.21%	6 Missing ⚠️
src/rules/coloring_qubo.rs	0.00%	1 Missing ⚠️
src/rules/ilp_qubo.rs	0.00%	1 Missing ⚠️
src/rules/ksatisfiability_qubo.rs	0.00%	1 Missing ⚠️
src/rules/maximumindependentset_qubo.rs	0.00%	1 Missing ⚠️
src/rules/maximumsetpacking_qubo.rs	0.00%	1 Missing ⚠️
src/rules/minimumvertexcover_qubo.rs	0.00%	1 Missing ⚠️
... and 1 more

❌ Your patch check has failed because the patch coverage (84.80%) is below the target coverage (95.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #69      +/-   ##
==========================================
- Coverage   96.36%   95.96%   -0.41%     
==========================================
  Files         197      197              
  Lines       27210    27676     +466     
==========================================
+ Hits        26222    26560     +338     
- Misses        988     1116     +128     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.