feat(debug): REDIS_LUA_DEBUG build flavor with in-VM Lua debugger

[fatal10110]

Adds a second WASM flavor (redis_lua.debug.{mjs,wasm}) carrying a full step-debugging surface while leaving the default binary byte-for-byte unchanged:

• Asyncify build with a single suspendable import, host_debug_request, defined in an Emscripten JS library (debug_library.js) so the paused script's Lua frames stay live while the JS host decides what to do.
• Pure-Lua debug agent (debug_agent.lua, embedded via a generated header) owning breakpoints, step modes, stack/scopes/variables inspection and paused-frame evaluate; C owns only the combined fuel+line hook, agent lifecycle, and teardown back to the fuel-only hook.
• eval_debug export loads the user chunk under the script SHA so debugger source identity matches; keeps debug/setfenv/getfenv and compiles ldblib.c in this flavor only.
• Line events burn fuel too: count events landing inside the (hook- disabled) agent are swallowed by the VM, so count alone cannot kill runaway loops under the debugger.
• JS API: load({ debug: true }), engine.evalDebug(), RedisHost. onDebugRequest, default debug asset paths; Asyncify-wrapped exports (arity 0) are treated as sret in the ABI sniffing.