fix(capi.lua): make the Lua C dependency namespace exact

[Sunrisepeak]

Problem

The immutable mcpplibs.capi.lua@0.0.3 source manifest declares bare lua = "5.4.7". The current canonical provider is compat.lua, while exact-selector mcpp maps an omitted namespace only to mcpplibs; consumers such as openxlings/xlings therefore fail instead of guessing between compat.lua and mcpplibs.capi.lua.

Proposed fix

Describe the existing 0.0.3 payload with the index's Form-B build metadata and declare compat.lua@5.4.7 explicitly. Keep the release URLs and immutable SHA256 unchanged.

Acceptance

• descriptor lint/parse succeeds;
• a local-index consumer builds and links mcpplibs.capi.lua@0.0.3 with compat.lua@5.4.7;
• openxlings/xlings builds with its root compat.ftxui correction and no candidate fallback.

Related: openxlings/xlings#520, mcpp-community/mcpp#398, mcpp-community/mcpp#400