language: add canonical nan literals and f32/f64 NAN constants

Status after the refactor

PR #125 completed the target-dependent half of the original issue: usize and isize now expose MAX, MIN, and BITS, with 32-bit/64-bit and engine coverage. That work is complete and is no longer part of this issue.

The remaining work is a source spelling for canonical floating-point NaN values and ordinary standard-library NAN constants.

Current limitation

Silk's decimal floating-point literal grammar cannot produce an IEEE NaN bit pattern. The literal-only constant-initializer rule (SEM0086) also correctly rejects computed or call-based workarounds such as 0.0 / 0.0 and f64.fromBits(...) in a constant declaration.

Runtime NaN values are already possible, so this is API and language-spelling work rather than a blocker for floating-point execution.

Decided design

• Add a dedicated, contextually typed floating nan literal.
• The literal defaults to f64 and becomes f32 in an f32 context.
• It denotes the established positive canonical quiet NaN for the selected width.
• Do not add sign or payload syntax.
• Declare pub const NAN: f32 = nan and pub const NAN: f64 = nan in ordinary standard-library source.
• Do not recognize the standard-library declarations by name in the compiler.
• Keep the literal-only constant-initializer rule; do not introduce general constant evaluation.
• Synchronize the authoritative constant specification with the already accepted target-dependent-constant delta while updating it for nan.

Out of scope

• General constant evaluation.
• User-selected NaN signs or payloads.
• Further target-dependent integer-constant work; PR #125 completed that scope.

Acceptance criteria

• The lexer/parser and semantic model accept a contextually typed nan literal.
• An unconstrained nan defaults to f64; an f32 context produces f32.
• f32.NAN and f64.NAN exist as ordinary standard-library constants.
• Tests show each constant is not equal to itself.
• Tests assert the positive canonical quiet-NaN bit pattern for each width.
• The evaluator, native LLVM backend, and direct Wasm backend agree.
• The OpenSpec/main specification records the literal and synchronizes the target-dependent constant rules.
• SEM0086 continues to reject non-literal constant initializers.

Start with the lexer/parser and semantic model entry points for numeric literals, then trace evaluation through the native LLVM backend and direct Wasm backend. Add the standard-library f32.NAN and f64.NAN declarations and update the OpenSpec/main specification. Done means contextual typing, canonical bit patterns, SEM0086 coverage, and agreement across all listed evaluators and backends.