docs: add repository refactor audit identifying repetition hotspots and ~2.3k LOC savings

[blackopsrepl]

Motivation

• The codebase contains multiple high-confidence repetition clusters (near-duplicate test suites, hand-expanded arity files, and higher-arity incremental constraints) that increase maintenance burden.
• The goal is to propose safe, high-ROI refactors that remove duplicated source while preserving SolverForge's zero-erasure and performance constraints.

Description

• Add docs/repo-refactor-audit.md containing a methodical duplication audit that identifies five high-yield refactor tracks and conservative LOC savings per track.
• The document proposes concrete changes for duplicated test trees, n-ary stream macro consolidation, higher-arity incremental scaffolding, and termination-counter normalization, and estimates a total savings of ~2338 LOC (range ~2.1k–2.6k).
• The document also includes a recommended phased rollout and guardrails to preserve hot-path monomorphization and runtime behavior.

Testing

• Ran repository enumeration with rg --files and targeted rg queries to locate candidate duplicate test paths, which completed successfully.
• Executed Python analysis scripts that computed per-file LOC, pairwise normalized similarity, and family LOC totals (used to produce the proposed savings estimates), which completed successfully.
• Verified the new docs/repo-refactor-audit.md content was written and readable (nl/file display), confirming the document was added and contains the audit and recommendations.

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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.