Exact symbolic growth, unified reduction size contracts, and deterministic solver backends

[isPANN]

Closes #1075, #1078, #1076, #788, and #1090.

Supersedes #1083 with a clean, function-oriented history built from the final code tree against main.

Summary

Exact symbolic reduction contracts

• Adds a shared exact expression core with integer and rational arithmetic.
• Adds conservative symbolic growth analysis for exponential, polynomial, logarithmic, factorial, and multivariable expressions.
• Introduces one rule-owned size relation per reduction, with exact, upper-bound, and explicitly unavailable fields.
• Enforces strict solution-extraction and numeric boundary contracts.
• Keeps path enumeration deterministic and reports symbolic and measured path sizes without coupling size algebra to route selection.

Registry-owned models and deterministic solving

• Moves construction schemas, validation, random generation, serialization, and variant metadata into the model registry.
• Registers native and fixed reduction-to-ILP solver workflows by exact source variant.
• Selects backends deterministically and distinguishes infeasible instances from backend failures.

CLI and MCP

• Uses the same registry contract for creation, inspection, solving, reduction paths, and size reporting.
• Keeps CLI and MCP outputs consistent and covers error paths and saved-route round trips.

Build and documentation

• Updates the paper, user documentation, and maintainer workflows for the final contracts.
• Uses the required HiGHS backend and verifies macOS ARM64, Windows x86_64, and RISC-V builds.

Commit structure

1. Unified symbolic reduction, registry, and solver compile-time contracts.
2. Registry-driven CLI and MCP workflows.
3. End-to-end reduction examples and integration coverage.
4. Documentation and maintainer workflows.
5. Cross-platform build verification.

Quality review

• Checked DRY, KISS, cohesion/coupling, CLI/MCP HCI, and semantic test coverage.
• Verified that discarded PR-only designs do not appear in the new branch history.
• Fixed the REST PR-search fallback test contract discovered during the review.

Verification

• make check
• python3 scripts/test_pipeline_checks.py
• Each code-bearing commit compiled independently in a detached worktree.
• The migrated tree matches the preserved final snapshot except for the intentional pipeline test fix above.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.29%. Comparing base (cb54ea0) to head (9760c9a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1137      +/-   ##
==========================================
- Coverage   98.00%   97.29%   -0.72%     
==========================================
  Files        1044     1053       +9     
  Lines      107366   115569    +8203     
==========================================
+ Hits       105228   112446    +7218     
- Misses       2138     3123     +985     

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