Enhancement: Clara v2 reimplementation (enum-indexed, Unbounded values)

[abitofhelp]

Reimplement Clara with a clean architecture:

• Enum-indexed declarative pattern (not procedural builder, not generic child packages)
• Unbounded_String for values, bounded for metadata
• Dynamic vectors for multi-value storage
• Testable Parse (accepts argument array, not just Ada.Command_Line)
• No SPARK — Clara requires an OS; SPARK is inappropriate
• Utility library structure (not full DDD layering)
• Explicit help — user provides Show_Help callback

Only consumer today is adafmt (3 commands, 1 flag, ~64 options, 1 positional).
Nothing is released, so no backward compatibility needed.

Resolves the STORAGE_ERROR (stack overflow) caused by bounded strings
(Max_Value_Length = 4096) in fixed arrays.

Reference: clara/exports/latestexport.txt