refactor: use upstream async runtime fixes

[dbrattli]

Actor-owned lifetimes carry Python lock, cancellation-registration, timer and delayed-continuation workarounds for defects in the older Fable runtime. This change uses the fixes released in Fable 5.20.1 while preserving actor shutdown, request/reply settlement and cleanup behavior.

• Use standard lock, disposable CancellationToken.Register, Async.Sleep and async.Delay; remove the corresponding Python platform helpers and cancellation guards.
• Restore unit-valued continuations in the lifecycle tests and observation barrier. Normalize erased unit success callbacks at the remaining Python context adapters.
• Keep the actor finalizer and shared-trampoline adapters, pending settlement gates and idle-mailbox cancellation wakeup. These enforce actor-specific guarantees that the upstream fixes do not provide.
• Add a regression requiring a pre-cancelled lifetime registration to invoke its callback exactly once.
• Update the compiler pin to Fable 5.20.1, the root and Timeflies Python runtime requirements to fable-library >=5.20.1, and both lockfiles to 5.20.1. The lifecycle ADC records the minimum fixed compiler/runtime version.

Upstream prerequisites

All required fixes are merged and included in Fable 5.20.1:

• Fable #5035: shared Python locks.
• Fable #5036: cancellation registrations.
• Fable #5037: unit continuations.
• Fable #5038: cancelled delays and finalizers.
• Update the compiler, both Python runtime requirements and both lockfiles to the released dependencies.
• Run the normal four-target suite, BEAM library build, formatting and packaging with the released dependencies.
• CI passes with released dependencies, including the Python 3.12 suite.

Validation

Executed using the released dotnet fable 5.20.1 and PyPI fable-library 5.20.1, with no local compiler or runtime overrides:

• .NET: 63 passed.
• Python 3.14: 61 passed, including all nine tests that failed in the previous 5.20.0 CI run.
• JavaScript/Node: 61 passed.
• BEAM: 53 passed; the generated library also compiles with rebar3.
• just check, Fantomas 8.0.8 --check, git diff --check and NuGet packing passed.
• Verified the installed Python runtime version/path, generated native registration and unit deadline callbacks, and packed Fable source containing the runtime requirement ADC.

Fantomas reports the existing unsupported .editorconfig setting; BEAM compilation reports existing unused-result warnings. Neither prevents validation.