refactor: extract partial eval loop unrolling

[lbliii]

Summary

• extract static for-loop transformation and unrolling into a phase-owned internal mixin
• move compile-time loop properties and iteration-context construction with that phase
• keep PartialEvaluator slot-only and concrete while sharing the existing 200-item bound with list-comprehension evaluation

Progresses #143.

Steward Notes

• consulted: compiler and testing stewards
• risk: MRO or state-layout drift in a compiler hot path, plus semantic drift in scoped loop contexts
• evidence: the mixin has no slots/state, PartialEvaluator remains concrete with no instance __dict__, focused and stability suites pass, and isolated free-threaded benchmarks show no regression
• unresolved tradeoffs: call/def inlining and the remaining control-flow/expression phases stay in Split partial evaluation into smaller phase-owned modules #143

Verification

• 319 passed in the focused partial-evaluation suite
• make verify-stability (4,459 passed, 5 skipped)
• ruff check, ruff format --check, and ty check clean
• Python 3.14.2 free-threaded isolated before/after benchmarks: compile-pipeline medians within noise or faster; partial-evaluation medians within noise or faster

Collateral

• no docs/changelog/API collateral: internal behavior-preserving phase extraction
• no benchmark baseline update: no reproducible regression

[Copilot]

Pull request overview

This PR refactors partial-evaluation static for-loop unrolling by extracting the transformation logic into a dedicated, phase-owned internal mixin module, while keeping PartialEvaluator concrete and slot-based. This aligns with issue #143’s goal of splitting partial_eval.py into smaller, behavior-scoped units without changing the public entrypoint.

Changes:

• Extracted static for-loop transformation + unrolling into LoopUnrollingMixin (partial_eval_loops.py) and made PartialEvaluator inherit it.
• Moved compile-time loop-context properties (loop.*) into the new module (LoopProperties).
• Centralized the 200-iteration unroll bound so both loop unrolling and list-comprehension evaluation share the same limit (MAX_UNROLL).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/kida/compiler/partial_eval.py	Switches PartialEvaluator to inherit loop-unrolling behavior from the extracted mixin and shares the unroll bound with list comprehensions.
src/kida/compiler/partial_eval_loops.py	Introduces an internal mixin implementing static for-loop unrolling and compile-time loop.* properties.