feat(lua): Python-style traceback + locals capture

[skhaz]

Python-style stack-trace + locals capture for Wippy Lua scripts, on demand.

Why

Lua scripts had no way to capture a stack trace with local variables — only a bare debug.traceback string with no locals, and even that was unavailable because SkipOpenLibs excluded the debug library entirely.

What

• Read-only debug subset as a cached global (cached_libs.go): traceback, getinfo, getlocal, getupvalue. Stripped: setlocal, setmetatable, setupvalue, and getmetatable (the last bypasses __metatable — a sandbox hole).
• traceback module (runtime/lua/modules/traceback):
  • traceback.format(level, opts) — formatted multi-line string (Python traceback.format_exc style).
  • traceback.frames(level, opts) — structured frames with locals and upvalues.
  • Options: depth, locals, upvalues, max_value_len, max_table_depth.
  • Cycle detection for self-referential tables; frames that panic under post-recover conditions are skipped instead of aborting the trace.
• Registered as a core ambient module (core_modules.go) — require("traceback") and global.

Usage

-- On-demand snapshot
print(traceback.format(1))

-- At the throw site (xpcall handler runs with stack intact)
xpcall(risky, function(e) return traceback.format(1) end)

Dependencies

Bumps github.com/wippyai/go-lua to a pre-release commit (wippyai/go-lua#32) for two prerequisites: xpcall catching errors under direct PCall/DoString, and inspect.GetStackFrame capturing upvalues. Once wippyai/go-lua#32 merges and tags a release, the pseudo-version can be replaced with the tag.

Testing

• TDD: RED → GREEN for the module, the cached-lib subset, and both fork fixes.
• make test-runtime, make lint, gofmt, vet — green.
• Mutation: 65% efficacy, 100% coverage; correctness-critical mutants killed.

Sample output (xpcall throw-site capture)

[3] <string>:6 (risky)
    Locals:
      depth = 0
      context = processing layer 0
      payload = {secret = shh, id = 0}
    Upvalues:
      risky = <function>

No visual/UI changes (runtime-only feature).