Parser: `decimal(p,s)` type not parseable despite AST and typechecker support

[mjm918]

Description

NamlType::Decimal { precision, scale } exists in the AST (ast/types.rs:43) and the typechecker maps it to Type::Float (infer.rs:1778, mod.rs:831), but the parser has no branch to parse decimal(p,s) syntax. The lexer also lacks a Keyword::Decimal variant.

As a result, decimal is lexed as an identifier and falls through to parse_named_or_generic_type, causing a type mismatch error.

Steps to Reproduce

fn main() {
    var xx: decimal = 1.0;
}

Expected: Parses decimal as a type (or decimal(10,2) with precision/scale)
Actual: type mismatch: expected float, found Spur(3)

What exists

• AST: NamlType::Decimal { precision: u8, scale: u8 } in namlc/src/ast/types.rs:43
• Typechecker: NamlType::Decimal { .. } => Type::Float in infer.rs:1778 and mod.rs:831

What's missing

1. Lexer: No Keyword::Decimal variant — decimal is lexed as an identifier
2. Parser: No branch in parse_type() (parser/types.rs:21-46) to handle decimal or decimal(p,s)

Fix

1. Add Decimal to the Keyword enum in the lexer
2. Add a parsing branch in parse_type() for Keyword::Decimal
3. Parse optional (precision, scale) params — e.g. decimal defaults to decimal(10,2), decimal(18,6) sets explicit precision/scale