Add with variant construction syntax #99

Goal

Add with as the canonical data-variant construction separator.

let failed = Result::Failed with "404 not found";
let ok = Result::Ok with {
    value = 42,
};
let pending = Result::Pending;

Source rules

• Data variants construct as Enum::Variant with <value>.
• Payloadless variants remain Enum::Variant.
• with on payloadless variants is invalid.
• Missing with for data variants is invalid once this syntax is adopted.
• Variants remain cases of one nominal enum type. They are never types, structs, functions, or callable constructors.
• Enum::Variant(...) remains invalid.
• Contextual .Variant construction and generic argument inference remain separate work.

Compiler scope

• Parse with after fully named variant paths.
• Support scalar, composite, call, array, parenthesized, and braced payload expressions after with.
• For named payload schemas, support with { field = value } and keep declaration-order field validation.
• Preserve current struct literal syntax: .Point{ ... } and .{ ... }.
• Reuse existing resolved construction evidence and shared VariantMake lowering.
• Do not add a second constructor path or re-resolve variants after typechecking.
• Update parser recovery for missing payloads, extra payloads, ambiguous control-header boundaries, and invalid call-style variants.
• Update language-support and tree-sitter grammar after compiler syntax settles.

Validation

• Parser and recovery tests for assignments, returns, calls, arrays, nested expressions, and control headers.
• Typechecker tests for generic/non-generic/imported variants, exact field validation, payloadless rejection, call-style rejection, and variants in type position.
• HIR, MIR, LLVM tests proving existing VariantMake path is reused.
• LSP hover/completion/diagnostic tests.
• Positive and negative x_test fixtures with bundled build/bin/peeper.