[ITER-2] Add numerical regression safety net for legacy core

[seonghobae]

Goal

Protect historical FIPC numerical behavior with machine-checkable regression fixtures.

Deliverables

• Add fixed fixtures and expected outputs for representative linking scenarios.
• Add tolerance-based regression tests in testthat.
• Wire regression suite into CI.

Exit criteria

• Regression test fails on intentional drift.
• Regression test passes on baseline.
• Any PR touching R/aFIPC.R includes regression evidence.

[coderabbitai]

🔗 Related PRs

seonghobae/eldery-journal-work#39 - solve_lp 방향 검증 추가 및 병렬 전역 변수 테스트 강화 [merged]
ContextualWisdomLab/kaefa#30 - Add GitHub Actions CI workflow for R CMD check [merged]
#5 - feat: add baseline package tests and private-safe security audit [closed]

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