chore: drop the BEAM workarounds for upstream Fable bugs

[dbrattli]

Draft — blocked on an upstream Fable release. Do not merge yet.

Stacked on #37, which is where both workarounds are introduced. Base will retarget to main automatically once #37 merges.

Removes the two BEAM workarounds that exist only to paper over Fable bugs. Both are now fixed on Fable's main, but neither is in a release yet — and CI runs dotnet fable (currently 5.11.0), so merging this today turns CI red.

What this removes

workaround	why it existed	fixed upstream by
test-beam flatten step in the Justfile	Fable wrote package modules to fable_modules/<pkg>/<subdir>/src/, which rebar3's project_app_dirs never scans — they compiled but were undef at runtime	#4817 — modules now go to <pkg>/src/
#if FABLE_COMPILER_BEAM in test/shared/Helpers.fs	Async.AwaitTask compiled to fable_async:await_task, which did not exist; the [<Emit("$0")>] identity stood in for it	#4819 — adds await_task(Task) -> Task., the same identity

Net effect: Helpers.fs has no target fork at all, and test-beam is back to compile → rebar3 → run.

Verification

Green against a local Fable checkout carrying both fixes:

just dev=true test
  Python      50 passed (50)
  JavaScript  43 passed | 2 skipped (45)
  BEAM        37 passed | 8 skipped (45)

Red against released Fable 5.11.0, exactly as expected — Python and JS pass, BEAM dies on the missing Parchment sink:

just test
  Python      50 passed (50)
  JavaScript  43 passed | 2 skipped (45)
  BEAM        {undef,[{fable_async,run_synchronously,2, ...
              error: Recipe `test-beam` failed with exit code 1

(The undef surfacing as fable_async:run_synchronously rather than the real call site is itself an upstream artefact, fixed by #4818 — the async re-raise used to discard the original stacktrace.)

Merge checklist

• test: migrate the shared suite to Scriptorium (Quill + Nib) #37 merged
• A Fable release containing #4817 and #4819 is out
• Justfile / CI bumped to that release
• just test green on the released compiler

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