Add repetition audit and refactor proposal (est. 3.1k–3.56k LOC savings)

[blackopsrepl]

Motivation

• Provide a single, repo-wide audit that identifies repeated code patterns and concrete refactor tracks to reduce duplication while preserving SolverForge architectural constraints (zero-erasure, ownership model).
• Give maintainers an ordered, low-risk plan with conservative and upper-bound LOC savings estimates to guide follow-up refactors.

Description

• Add docs/repetition-audit-2026-04-20.md containing the audit methodology, six prioritized refactor tracks (duplicate test suites, weighting DSL unification, arity macro consolidation, higher-arity incremental consolidation, selector enumeration kernels, shared test fixtures), and conservative/upper LOC-saving estimates (~3,100 to ~3,560 LOC).
• The document includes recommended execution order and risk controls that preserve hot-path constraints and require make test/benchmarks after each slice.
• This change is documentation-only and does not modify runtime code or public APIs.

Testing

• No unit or integration tests were required for this documentation-only change; repository verification steps were executed and completed successfully, including the duplication scan (python sliding-window script), quick file checks (rg / wc -l) and creating the commit, all of which succeeded.

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Codex Task

[blackopsrepl]

Closing in favor of the consolidated repetition-reduction plan in the repo-root PRD. This branch is based on v0.8.9 and is 19 commits behind current main, so it should be treated as input only rather than rebased forward.