Lower core F# expressions, patterns, declarations, and exceptions

Parent: #8
Blocked by: #33 and #34

Outcome

Implement target-agnostic lowering for the core batch-language families.

Acceptance criteria

• Values, functions, applications, locals, mutation, control flow, pattern matches, decision trees, records, unions, classes, structs, interfaces, object expressions, closures, generic operations, and exceptions lower from typed input.
• Closure conversion, captured mutation, tail positions, exception regions, and debug anchors are explicit.
• Lowered output uses stable symbolic operands and identities rather than metadata handles or final layout.
• Each language family runs through parse, typecheck, lower, emit, load, and execute using small compiler-owned programs.
• Behavioral comparison permits different valid IL while preserving public API, runtime, exception, and debug semantics.
• Unsupported families fail before successful publication rather than generating placeholder behavior.

Decision inputs

Issues #5 and #7; ADRs 0006, 0008, 0015, and 0021.

This issue inherits the corpus-neutral acceptance policy in #8.

Start with parent issue #8 and unblockers #33 and #34, then read decision inputs #5 and #7 plus ADRs 0006, 0008, 0015, and 0021. Trace the parse, typecheck, lower, emit, load, and execute stages using compiler-owned programs. Done means the listed language families lower with explicit closure, mutation, exception, tail-position, and debug semantics, while unsupported families fail before publication.