Remove mutability tracking from MIR semantics

Motivation

PR #948 removed the mutabilityOf(...) ==K mutabilityMut guard from #setLocalValue, because MIR's LocalDecl::mutability is a source-level annotation (let x vs let mut x), not an assignment constraint. The Rust compiler's borrow checker validates assignment legality before emitting MIR, so all assignments in well-formed MIR are valid regardless of the mutability field.

After #948, #forceSetLocal is functionally identical to #setLocalValue — both write without checking mutability.

Proposal

Remove mutability tracking from the MIR semantics definition entirely:

• Remove mutabilityMut / mutabilityNot from typedValue and related data structures
• Unify #forceSetLocal into #setLocalValue (or remove #forceSetLocal)
• Clean up any remaining mutability checks or propagation throughout the codebase

References

• PR #948: fix(rt): remove mutability guard on local variable assignment
• LocalDecl::mutability docs: "Whether this is a mutable binding (i.e., let x or let mut x). Temporaries and the return place are always mutable."