Extract the assembler into a reusable xasm library

[epi]

Summary

Refactors xasm so the assembler core can be used as a library (libxasm) from any D program, not just through the xasm command-line tool. xebin is going to be its first user.
All the assembly logic previously living in source/app.d now sits behind an Assembler class in a new source/xasm/package.d module. app.d becomes a thin CLI frontend that drives that class.

What changed

• New xasm module (source/xasm/package.d): the assembler implementation, moved out of app.d verbatim and wrapped in an Assembler class with a private implementation and a small public surface.
• I/O is fully decoupled via delegates — the library performs no direct file access or console output. Callers supply:
  • SourceReader / BinaryReader — read source and binary-include files
  • DiagnosticSink — receive warnings and errors as structured Diagnostic values (Severity, filename, line, source line, message)
  • ListingSink — receive the assembly listing and label table line by line
• Object code is staged through a buffer, exposed via assembler.object, instead of being written straight to a file.
• Public API documented with ddoc; make libxasm.html generates the library reference. Label is now a struct, and the module carries a documented usage example (runnable as a unittest).

Notes

• No changes to the assembler's behavior or output for CLI users; this is a structural refactor only.
• Man page HISTORY section notes that xasm can now be used as a library, without assigning a concrete version number.