feat(core): rxc archive format with multi-file support

[deepracticexs]

Summary

• RXC now uses tar.gz format internally, enabling multi-file resource support
• FolderLoader supports any file names and nested directories (no longer requires content file)
• Added modern-tar dependency for tar packaging

API Changes

New:

// Create from files
await createRXC({ content: "Hello" })           // single file
await createRXC({ 'a.ts': '...', 'b.css': '...' }) // multi-file  
await createRXC({ archive: tarGzBuffer })       // from archive

// Read files
await rxc.file('content')  // → Buffer
await rxc.files()          // → Map<string, Buffer>

Removed:

• loadRXC() - use loadResource() instead
• rxc.text(), rxc.json() - use rxc.file('content')

Test plan

• All 57 BDD scenarios pass
• All unit tests pass
• TypeCheck passes
• Lint passes (warnings only)

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