Guard Lua provider admission and verify rooted coroutine dispatch - #90

Summary

Reject a host-provided Lua state with a negative stack top before stamp verification, and reject stamp verification that changes that top. Both paths release operation admission through the existing exception cleanup; they do not reset the stack or retry the provider.

Add managed boundary tests for negative or changed stack-top observations, with invariant failure diagnostics. Add real Lua 5.3 fixtures that preserve existing stack values and exercise rooted worker/main coroutine dispatch, repeated success and exception paths, and queued-stop cleanup.

Scope and ownership

SDK Lua admission and Hosting regression fixtures only. No Client workflow, native bridge, dependency, or public API changes. The guards contain invalid provider state; the original released-bundle dispatcher failure remains unexplained and is not claimed fixed.

Validation

The earlier live dispatcher failure did not reproduce during the diagnostic replay. Plugin reload remains blocked by the private Settings bridge's empty plugin checklist. Neither observation establishes full SDK host qualification.

Compatibility and release impact

No public API change. Invalid provider states fail earlier with an InvalidOperationException; existing admission cleanup is preserved. Valid caller-owned stack values remain intact. CHANGELOG.md records the behavior under Unreleased. Installed and released binaries are unchanged.

Checklist

• The change is focused and follows CONTRIBUTING.md.
• Relevant regressions pass with no skipped tests.
• Consumer-visible behavior is recorded under Unreleased.
• No public API or dependency changes require API files or lock regeneration.
• Documentation and claims distinguish fixture evidence from exact-host qualification.
• Exact-host Q19 qualification and same-process plugin reload are still outstanding.