Implement core F# type inference, unification, and generalization

Parent: #8
Blocked by: #31

Outcome

Implement compiler-owned type inference for core F# expressions and declarations.

Acceptance criteria

• Inference variables, unification, substitutions, annotations, generic instantiation, let-generalization, value restriction, tuple and function types, and recursive bindings are represented explicitly.
• Expression and pattern checking covers bindings, functions, application, control flow, matches, collections, and generic calls within declared feature rows.
• Nullness and language-version behavior are option inputs rather than project-specific switches.
• Positive cases produce deterministic typed results; negative and ambiguous cases match supported Compatibility Oracle diagnostics.
• The checker uses metadata-only reference symbols and never executes target code.
• Typed output preserves source ranges, coercions, instantiations, and dependencies required by later lowering and incremental work.

Decision inputs

Issues #4 and #5; ADRs 0005, 0014, and 0018.

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

Start with parent issue #8 and unblock dependency #31, then read issues #4 and #5 plus ADRs 0005, 0014, and 0018. Use the acceptance criteria as the completion checklist: core inference and checking must cover the declared feature rows, produce deterministic typed results and compatible diagnostics, and preserve the metadata needed by later stages. No source files or tests are named.