What this does

Adds an --accept-style escape hatch to the golden harness. When a codegen or
optimizer change legitimately moves the generated IR or Lua, you can rewrite the
affected goldens in place instead of deleting them by hand:

PSLUA_GOLDEN_ACCEPT=1 cabal test spec

A mismatching golden is overwritten with the actual output and the test passes,
the same way tasty-golden --accept does.

The safety asymmetry

Acceptance is opt-in per golden. The harness gains acceptableGolden, a variant
of defaultGolden that carries an acceptable :: Bool flag:

• golden.ir and golden.lua are derived: a pure function of the code under
  test. They use acceptableGolden, so PSLUA_GOLDEN_ACCEPT may rewrite them.
• eval/golden.txt is the hand-verified semantic oracle. It stays on
  defaultGolden (acceptable = False), so it is never auto-accepted. A
  program-output change still fails the run until you fix the regression or
  update the oracle on purpose.

This is the guard scripts/golden_reset lacks: that script deletes every
golden.*, the eval oracle included, which is how a past regression slipped
through unnoticed. PSLUA_GOLDEN_ACCEPT only touches the structural goldens.

Docs

Adds docs/GOLDEN_TESTING.md: what each artifact is, why the eval golden is the
oracle, the accept / reset / full-diff knobs, and how to add a new golden test.
Also adds a CLAUDE.md note that the golden harness re-implements the IR
pipeline in compileCorefn, so any new IR pass must live inside
optimizedUberModule or the goldens silently bypass it.

Scope

Test infrastructure only, no compiler or runtime change. Independent of #105
(the magic-do fix for #46); this branch is cut from main.

Verification

• cabal test spec: 233/0 on main plus this branch.
• Checked the asymmetry directly: perturbed both golden.ir and
  eval/golden.txt for one module, then ran with PSLUA_GOLDEN_ACCEPT=1. The
  golden.ir was accepted and rewritten back to the regenerated output (clean
  git diff), while the eval/golden.txt test still failed and the file was
  left untouched.
• fourmolu, hlint, nix fmt clean on the touched files.