Rework fibres runtime for coroutine-safe errors, scopes, and channel/waitgroup semantics

[lePereT]

What type of PR is this? (check all applicable)

• Bug Fix
• Code Refactor
• Test

Description

Reworks the fibres runtime to be exception-neutral, coroutine-safe, and aligned with structured concurrency semantics.

• Introduces a local coxpcall shim providing coroutine-safe pcall/xpcall and coroutine.running semantics.

• Changes fibers.runtime to route uncaught fiber errors through an internal queue and wait_fiber_error, rather than aborting the process.

• Refactors fibers.scope to:

  • supervise fibers via the runtime error queue,
  • make scope.run return (status, error, ...) instead of raising,
  • enforce that Scope:spawn only runs on a running scope,
  • drive scope closure, defers and status changes via a join worker.

• Simplifies fibers.op:

  • removes wrap_handler and exception-based control flow,
  • makes events exception-neutral and adds or_else, finally, bracket and higher-level choice helpers (race, first_ready, named_choice, boolean_choice),
  • adjusts new_cond and nack signalling to run via the scheduler and coxpcall.

• Reworks fibers.channel to:

  • track per-operation entries and support cancellation via on_abort,
  • skip cancelled senders/receivers when matching operations.

• Reimplements fibers.waitgroup with per-generation conditions so waitgroups can be reused safely.

• Tightens fibers.performer.perform with an explicit Event metatable check.

• Extends syscall errno exports with EBADF and ENOENT.

• Updates tests (test_channel, test_cond, test_op, test_scope, test_waitgroup) to reflect the new semantics and add coverage for waitgroup reuse and scope cancellation paths.

Manual test

• yes

Manual test description

Ran the Lua test suite locally. All completed without failures.

Added tests?

• yes

Added to documentation?

• no documentation needed