Support constant-backed type aliases in variable declarations (safe + unsafe) with explicit invalid-case errors

[Copilot]

This change adds first-class support for using ALL_CAPS constants that hold type values as variable type aliases (e.g. n: BYTE = 3 after BYTE: type = u8). It also hardens invalid alias paths with explicit ProgramErrors for non-constant alias names and non-type alias values.

• Type alias resolution in variable declarations

  • Variable declaration type checking now resolves declared type names through in-scope alias bindings (locals/slots) when present.
  • Resolved aliases are used for validation and union wrapping, preserving existing behavior for non-alias type names.

• Explicit alias validity rules and error messages

  • If an alias binding is found but the name is not ALL_CAPS, a clear ProgramError is raised with constant-declaration guidance.
  • If an ALL_CAPS alias exists but does not hold a type value, a clear ProgramError is raised with expected declaration format.

• Type-literal parsing support for unsafe keywords

  • Expression parsing now treats unsafe numeric type keywords (i8/i16/i32/i64/u8/u16/u32/u64/f32/f64) as type literals, enabling declarations like BYTE: type = u8.

• Demo coverage updates

  • Added a new type-alias demo set:
    • valid alias usage,
    • invalid non-constant alias usage,
    • invalid non-type alias value usage.
  • Extended valid demo coverage to safe aliases: int, float, text, bool.

BYTE: type = u8
WHOLE: type = int
WORD: type = text

n: BYTE = 3
a: WHOLE = 42
c: WORD = "ok"