Make Lambda Solved unification iterative

[jaredramirez]

Summary

Overnight I kicked off codex saying "make unification not recursive" & it interpreted it as lambda solve unify, not type checker unify lol, but now that this is done, there's no harm in keeping it, so here we are

• Convert Lambda Solved type unification from recursive child descent to an explicit reusable work stack.
• Preserve exact active-pair cycle handling and representative-link semantics with process/finish frames.
• Add a structural postcheck unit test that rejects recursive calls inside the unification loop.

Test Plan

• zig build run-test-zig-module-postcheck
• zig build run-check-postcheck-architecture
• zig build minici starts successfully but is blocked in build-ci by build-coverage-tools: kcov_system_lib cannot find dynamic system libraries z and curl in the Nix macOS SDK paths. Reproduced with zig build build-ci --summary all --color off and zig build build-coverage-tools --summary all --color off.