docs: reposition around Lua 5.1 interop rather than general scripting

[mschmicking]

The README led with "embed Lua 5.1 in your Node.js programs" — which invites a comparison this package loses:

fengari          6,562,808 /mo   pure JS, no compiler
wasmoon            117,044 /mo   WASM, no compiler
node-lua-runner         40 /mo   compiles from source

But that is the wrong comparison. You reach for this when you need Lua 5.1 exactly — to interoperate with a runtime you don't control (firmware, Redis scripting, OpenResty, game modding) — or when you need Lua to touch the real filesystem and run real processes. No no-compile alternative can do either: they target a different Lua version and run sandboxed.

Changes

• Leads with the interop use case and names the ecosystems that are on 5.1, so someone searching for that problem can recognise it.
• Adds an honest "Is this the right package?" section that names wasmoon and fengari, quotes their own descriptions, and says plainly to use them when the Lua version doesn't matter. Sending the wrong users away costs nothing; the right ones can now find themselves.
• Documents the LuaJIT distinction. This is stock PUC-Rio Lua 5.1.5. Source and C API compatibility with a LuaJIT target are fine, but precompiled bytecode is not interchangeable. Anyone doing firmware interop needs to know that before depending on it.
• Notes that Node-API is ABI-stable across Electron as well as Node, which is what makes this usable in a VS Code extension without rebuilding per host release.
• Adds the untrusted-code warning to Caveats — the full stdlib including os.execute is the point of the package, and that has an obvious implication.
• npm description and keywords updated to match, since that is what shows in search results.

I deliberately did not assert version numbers for wasmoon or fengari — I could not verify those from the registry, so the table describes their approach in their own words and leaves the version claim as "a newer Lua than 5.1".

Verified

• README 100 → 137 lines, all local links and anchors resolve
• package.json still at 2.0.1 (release-please's bump untouched)
• Tarball guard passes: 67 files, 29 vendored Lua sources

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