MagicDo: recognize Effect/ST chains by qualified name - #259

Closes #182.

MagicDo used to recognize Effect/ST do-chains by syntactic shape: unwinding application spines with fuel, chasing aliases, projecting fields out of dictionary literals, and speculatively beta-reducing. Any rewrite that changed a chain's shape could silently starve it, and a miss fell back to thunk-nested closures. This PR switches recognition to qualified names.

The change canonicalizes Effect/ST dictionary applications into applications of the real foreign methods (Effect.bindE/pureE, Control.Monad.ST.Internal.bind_/pure_). This is compile-time dictionary projection, exact by the instance definitions (bindEffect.bind = bindE and so on), and any residual magicDo never lowers stays executable at runtime. The rewrite runs in two tiers: at CoreFn translation (applications build bottom-up, so nested pairs compose; covers cross-module uses and the purs CSE floats) and as an optimizer rule (a same-module dictionary reference is Local until the linker requalifies it). Details in Note [Canonical Effect/ST heads].

MagicDo then matches the canonical heads through three lenses: a direct reference, the dissolved foreign-accessor read, and one top-level alias hop. The fueled unwinding, alias chasing, dictionary projection and speculative beta are gone, and with substituteMoveM out the pass is pure. New on top: a pure run peephole, App (pure x) EffectRunArg -> x, which collapses pure-terminated tails, mid-chain x <- pure e statements, and the pre-existing runs from the foreign lifter's run*Fn wrappers. Emission (chunking, EffectRunArg) and pipeline order are unchanged.

Two pre-existing miscompiles, found and fixed first

Two of the new golden modules tripped the checked pipeline's DuplicateBinder lint on main, and once compiling, one printed its effects twice. The branch therefore starts with two independent fixes, each demonstrated red first by a unit test:

• inlineLocalBinding's use-once path pasted an effect-run RHS (x = m $magicDoRun) into the Let body. DCE deliberately keeps such statements even when unreferenced, so the paste left two copies and the effect executed twice at runtime. Golden.TailRecM2Shadow was affected on main: its main re-ran the whole tailRecM machinery inside the log argument, on top of the kept binding of the same computation.
• freshenBinders renamed all Let-bound names through one name-keyed map. A pasted magicDo thunk binds several statements to the GUC-exempt discard binder _, so they all collapsed to the same fresh name, and the checked pipeline failed with DuplicateBinder in the specialize pass.

Commits

1. fix(optimizer): keep effect runs out of local inlining (the double-execution bug).
2. fix(ir): keep freshenBinders from collapsing discard binders.
3. test(golden): pin the missed shapes at current output (EffectPureChain, STDoBlock, MixedEffectSTDo, with hand-written eval oracles), so the feature's re-accept diff is its own demonstration.
4. feat(ir): the canonicalization + name-based MagicDo + unit specs (written red against the old code where they could compile).
5. test(golden): re-accept, net -1632 golden lines.

Verification

• Full suite green (853 examples), checked pipeline linting every pass boundary.
• Every eval/golden.txt oracle untouched and passing.
• Golden.LongDoBlock byte-identical. LongMaybe/Either/Writer/State/Stack bind goldens byte-identical; LongExcept/LongReader move by supply renumbering only (their mains are Effect-typed), so the discard rewrite did not leak into non-Effect monads.
• No golden gained a single closure, and no golden.lua keeps bindEffect/bindST/applicative* method-call residue in chain position.
• bench/ci fnew counters match the committed oracles.

Sample of the payoff, Golden.EffectPureChain (104 -> 30 lines, whole Effect dictionary web and the PSLUA_runtime_lazy fixture DCE'd away):

M.Golden_EffectPureChain_Test_count = function(n)
  return function()
    local _ = Effect_Console_log(...)()
    local x = n + 1
    local _ = Effect_Console_log(...)()
    return x + x * 2
  end
end

And Golden.UncurriedLift's sumST collapses to return a + b.

Follow-ups deliberately left out: flipping SkipCallSites pre-magicDo (now conservative caution rather than a recognition constraint) and lifting the Effect/ST core behind the canonical names (#228).