Unify scope boundaries and Ops semantics; document structured concurrency model

[lePereT]

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

• 🍕 Feature
• ✅ Test

Description

This PR strengthens the structured concurrency core and improves diagnostic consistency across Lua runtimes.

It includes two related changes:

1. Scope semantics and boundaries

• Updates fibers.scope to provide stable, predictable structured concurrency semantics for long-running, I/O-heavy systems.
• Establishes structural lifetime on join: attached child scopes are joined by the parent (including child finalisers) in attachment order.
• Introduces an explicit admission gate (close()), and ensures cancellation closes admission and cascades downward.
• Treats cancellation as a first-class termination mode distinct from failure, using a dedicated cancellation sentinel.
• Provides scope-aware op helpers with status-first boundaries (try/perform, join_op, run, run_op) and structured reports for child outcomes and finaliser errors.
• Ensures join/finalisation is non-interruptible by running in a join worker and using op.perform_raw.

2. Coroutine-safe xpcall traceback splicing (Lua 5.1 environments)

• Revises the local coxpcall implementation to make traceback output better match LuaJIT by:

  • using debug.traceback(co, ...) for the failing coroutine, and
  • splicing outer call-site frames by walking the coroutine parent chain with a synthetic xpcall boundary per hop.

• Preserves key semantics:

  • pcall continues to propagate the original error object unchanged;
  • xpcall(f, debug.traceback) continues to behave as expected;
  • handler faults are reported as handler faults.

Tests:

• Adds/updates scope tests covering structural join behaviour, admission close/cancel behaviour, cancellation vs failure accounting, and boundary helpers (join_op/run/run_op).

Manual test

• 👍 yes

Manual test description

• Exercised scope creation/spawn/cancel/join sequences in a small local harness (listener → child scope → nested work) and verified:

  • cancellation does not mark the scope failed;
  • join drains attached children and runs finalisers deterministically;
  • tracebacks include failing coroutine frames plus outer call-site frames under Lua 5.1-style coroutine-unsafe xpcall.

Added tests?

• 👍 yes

Added to documentation?

• 🙅 no documentation needed

[optional] Are there any post-deployment tasks we need to perform?

None

[corp0529]

magic strings here looks brittle, could we have a single source of truth for the function names and these finds, possibly constructing the table of functions in a single place. Otherwise, if function names change, this will break tracing silently.

[lePereT]

very good point! will think on what we can do here

[corp0529]

Could a bug introduce an infinite parent loop here? Should we guard against that?

[cpunt]

Do we not need this?

[cpunt]

Lgtm. Spent some time reading the comments, playing with the new semantics, and trying debug mode (super helpful). If @corp0529 is happy, I’m happy to get this in.