refactor(ir): extract unknown-type diagnostic pass into semantic_diagnostics (A2.4)

Moves the issue-0064 unknown-type pass (checkUnknownTypeNames + 11 helpers:
collectDeclaredTypeNames, harvestScopeDecls, checkStructFieldTypes,
checkFnSignatureTypes, checkScope, walkBodyTypes, checkCastTarget,
checkTypeNodeForUnknown, reportIfUnknownType, isBuiltinTypeName, isIdentLike)
out of Lowering into a new src/ir/semantic_diagnostics.zig (UnknownTypeChecker).

The checker holds borrowed references (alloc, *DiagnosticList, *TypeTable,
*ProgramIndex, main_file) — not *Lowering — and queries the canonical facts:
declared top-level names from ProgramIndex, primitives from
TypeResolver.resolvePrimitive, registered concrete types from the TypeTable.
The AST decl/scope walk stays (it collects LOCAL type decls, which ProgramIndex
doesn't track — a per-pass scope need, not a parallel authoritative list).

Lowering.lowerRoot builds the checker only when diagnostics are active and runs
it; the 12 functions are deleted from lower.zig. Barrel-wired in ir.zig.
Example snapshots (issue-0064 regressions 1111-1115) are the guard, matching the
checkErrorFlow precedent (no .test.zig).

Phase A2 complete. Gate: zig build, zig build test, run_examples 351/0.

src/ir/ir.zig

@@ -7,6 +7,7 @@ pub const lower = @import("lower.zig");
 pub const program_index = @import("program_index.zig");
 pub const type_resolver = @import("type_resolver.zig");
 pub const packs = @import("packs.zig");
+pub const semantic_diagnostics = @import("semantic_diagnostics.zig");
 
 pub const TypeId = types.TypeId;
 pub const TypeInfo = types.TypeInfo;