Introduce the opt-in Whale typed-HIR IR backend #861

Motivation

Alpha needs an initial Wave-to-Whale integration alongside the default LLVM backend. Previously the existing --whale flag was unimplemented. Related to #93; this is one component, not the migration tracker itself.

Scope

Root whale/ crate; Linux amd64 at O0; verified textual IR and check. Preserve HIR computation widths, ordered conversions, source diagnostics and input/output protection. Unsupported requests must fail without LLVM fallback. PR #859 records the locked upstream revision and validation.

Implementation status: Already merged in PR #859. This issue restores missing tracking; do not duplicate the implementation. No native execution or backend parity is claimed.

Completion criteria

• Provide check and build --emit=ir through the existing --whale flag.
• Cover scalar/control-flow lowering, ordered casts, diagnostics and the expression-depth boundary.
• Keep LLVM as the default and reject unsupported outputs and targets.