fix(beam): keep the original stacktrace when an async error is re-raised

[dbrattli]

Split out of #4817, where it rode along as an unrelated second fix.

Problem

An error inside an async computation does not propagate as an exception: it is caught where it happens and handed to the context's on_error as a bare term. By the time run_synchronously re-raised it, the original stacktrace was long gone, so erlang:error/1 built a fresh one and every async failure was reported at the re-raise line — a line whose only job is to re-raise.

before:  top frame: {fable_async,run_synchronously,2,[...,{line,147}]}
after:   top frame: {st_demo,boom,0,[{file,"st_demo.erl"},{line,4}]}

Fix

Each catch site in fable_async/fable_async_builder that hands an error onward parks the stacktrace it caught, and the re-raise restores it when it belongs to that error. Errors that crossed a process boundary, or that were rewritten by wrap_error, fall back to a fresh stacktrace.

A parked entry is tagged with the enclosing run_synchronously call's token, and only that call can consume it. Untagged, a stacktrace parked for an error that was later handled (a try_with whose handler succeeded) would linger in the process dictionary and could be restored onto an unrelated later error that happened to be an equal term — attaching a plausible but wrong stacktrace, which is the very failure this mechanism exists to remove. try_with additionally drops the parked entry on the paths where it handles an error rather than propagating it.

start_with_continuations deliberately does not park: the caller supplies OnError and receives the error term directly, so there is no re-raise to restore onto. That is now stated in a comment rather than left as a silent gap.

Tests

tests/Beam/erl/async_stacktrace_tests.erl — native Erlang rather than F#, because the assertions are about Erlang stack frames, which have no counterpart on .NET (the F# suite runs there first). Picked up automatically by erl_test_runner.

Each test discriminates against the relevant baseline:

	main	tagging removed	this PR
original stacktrace kept	❌	✅	✅
handled stacktrace not reused	✅	❌	✅
nested run keeps own	❌	✅	✅

Verification

In-tree BEAM suite on this branch: 2636 passed, 0 failed (2633 baseline + the 3 new tests), plus both entry-point program tests.

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