refactor: deduplicate interpolator precompute and code4 matrix construction

[henryiii]

🤖 AI text below 🤖

Part of the code review in #2706.

Summary

Deduplicates repeated code in src/pyhf/interpolators/:

• code1.py and code4.py: the bases_up/bases_dn/mask_on/mask_off (plus ones in code4) block was duplicated verbatim between _precompute and _precompute_alphasets. Extracted into a private _precompute_alphaset_dependent() helper called by both, keeping _precompute's extra histogram-only setup. Removed the redundant bare return statements at the ends of _precompute_alphasets.
• code4.py: the six-row A_inverse matrix literal appeared twice — once in code4.__init__ and once inside _slow_code4.product, where it was rebuilt on every scalar call. Factored it into a module-level _a_inverse(alpha0) function returning the nested list, used by both. In _slow_code4 it is now computed once per instance (alpha0 is fixed at construction) instead of per call.

Same math, same shapes, no behavior change.

Test plan

All run against the dev environment (strict pytest, filterwarnings=error), all backends:

• pytest tests/test_interpolate.py -q → 69 passed (includes slow-path vs fast-path agreement tests)
• pytest tests/test_validation.py tests/test_regression.py -q → 26 passed
• prek -a (lint/format) → clean

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: 081af9d1-195f-43ef-93b8-58d17291390d

📥 Commits

Reviewing files that changed from the base of the PR and between 0e54a51 and fa8c088.

📒 Files selected for processing (2)

• src/pyhf/interpolators/code1.py
• src/pyhf/interpolators/code4.py

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