[Perf] compiler: eliminate the --no-opt self-compilation memory floor & stack-unsafety (#40)

[katsujukou]

What & why

This PR partially resolves #40 by addressing all currently identified scalability problems
except those originating in the optimization passes. With these fixes, purs-wasm can now
compile itself to Wasm under --no-opt (running the self-compiled output is still gated on
ST support — see follow-ups).

Changes

• Stop retaining CoreFn, MIR, and ANF IR in memory through code generation.

  • CoreFn is dropped once translated to MIR (translate + lambda-lift fused per module).
  • MIR is dropped once lowered to ANF.
  • ANF is dropped once code generation to Binaryen completes.

  Previously, keeping all intermediate representations alive imposed a linear memory floor
  proportional to compilation progress — the front-half OOM that blocked self-compilation.

• Convert Effect impurification into a trampolined execution model (stack-safe on deep ASTs).

• Fix several secondary issues uncovered by --no-opt builds:

  • stripNewtype did not traverse as patterns (name@pat).
  • Eta-expansion for recursive let bindings failed to recognize functions built via
    mkFnN / mkEffectFnN (and let-wrapped / saturated point-free forms) as functions.
  • Multiple sources of stack-unsafety in code generation (genBody spine, genSwitch /
    genLitSwitch, addInternStr).
  • Stack-unsafety from traversing Array in raw Effect (whole-program emission loops).

  These interacted and collectively caused OOMs, hangs, and Maximum call stack size exceeded
  failures in --no-opt builds.

Fix uncovered while running the self-compiled compiler

• Externs.Decoder imported PureScript ADT constructors from within a foreign module, relying on
  the JS runtime representation of PS values. Reworked to pass the constructors in from the PS side
  (the ADR-0014 foreign-provider idiom). In general, foreign modules must not depend on the JS
  representation of PureScript values.

Follow-ups (not in this PR)

• ulib/arrays fromFoldable and the broader ST mutable-array support on wasm — tracked on a
  separate branch; the self-compiled compiler currently traps at the marshalling boundary for the
  Data.Array.ST / Control.Monad.ST foreign layer.
• The optimizer-pass scalability gate (the NbE reducer is exponential without sharing) — ADR 0035 / 0036.

Checklist

• Docs updated — docs/ + ADRs reflect the changes (frontiers, supported-features, ADR 0020 / 0035 / 0036).
• Tests — regression guards added for the lowering fixes (as-pattern, recursive-let) in the
  unit lane; the codegen / serialize stack-safety fixes are currently validated only by the
  self-compile, not by a routine-lane guard.
• Design changes have an ADR — ADR 0035 / 0036 added (forward-looking design for the
  optimizer gate); the changes in this PR are bug/perf fixes within the existing ADR 0009 model.
• No perf regression — bench bundles byte-identical; node bench/run.mjs <dir> all within
  noise; bench/snapshots/baseline.json not overwritten.
• [ ] N/A Runtime GC-type / ABI — no value-substrate / host-ABI change.