fix: preserve Go stack traces for coroutine panics

[eliotee]

Coroutine execution drops the Go stack trace when a Go callback panics, even with IncludeGoStackTrace enabled. Include the current Go stack in the recovered panic message before returning it through LState.Resume, coroutine.resume, or coroutine.wrap. Lua error objects retain their identity, including errors raised as *ApiError.

Fixes #529.

Validation:

• The regression test fails on the original code for all three coroutine entry points with stack traces enabled.
• Coverage includes both option values and Go panics, *ApiError panics, and Lua errors carrying table values.
• make build
• go test ./...
• go test -race -skip '^TestLocalVarFree$' ./...

The full race run detects an existing race in TestLocalVarFree: countFinalizers reads numActiveUserDatas while a finalizer updates it atomically. The same race reproduces in an unmodified checkout of the base commit (75f4976); all other tests pass with the race detector.