fix(0115): source-aware global selection — own-wins for module globals

The globals registry (global_names) was last-wins across modules with no
per-importer gate: any module's bare K could read/write/type against an
unrelated module's same-named global (hash.sx's K table hijacked every
user K once std's namespace tail pulled hash into the program), and an
own const of an unsupported shape borrowed another module's const and
panicked at the unresolved-type tripwire.

- var_decl joins RawDeclRef: module globals are selectable raw authors.
- selectGlobalAuthor (the globals analogue of F2's selectModuleConst):
  own author wins, one flat-visible author resolves, >=2 distinct flat
  authors diagnose loudly, authored-but-not-visible diagnoses, and a
  compiler-synthesized global (no raw author) emits untracked. A var_decl
  author whose per-source registration was deduped at flat-merge (two
  modules declaring the same extern symbol) serves the symbol's
  registration.
- All bare-identifier global sites route through it: value read, addr-of,
  assignment (store + compound), lvalue address, fn-ptr call, call param
  typing, and expression type inference.
- selectModuleConst gains .own_opaque: an own const author with no
  materialized per-source value (e.g. an array '::' const) blocks
  borrowing another module's same-named const — the read diagnoses
  cleanly instead of panicking.
- The fn-as-VALUE arm admits raw-facts-only authors: an own fn whose name
  a flat-merge collision dropped from the global decl list (first-wins)
  now resolves via author selection for func_ref/closure/Any shapes too.

Regressions: examples 0835 (own const vs flat array global), 0836 (main
const vs namespaced array global, incl. inference), 0837 (own array
const never borrows cross-module — clean unresolved).

src/ir/lower/decl.zig

@@ -1024,6 +1024,11 @@ pub fn globalInitValue(self: *Lowering, vd: *const ast.VarDecl, var_ty: TypeId)
                 const sel: SelectedConst = switch (self.selectModuleConst(id.name)) {
                     .resolved => |s| s,
                     .none => .{ .info = ci_global, .source = null },
+                    .own_opaque => {
+                        if (self.diagnostics) |d|
+                            d.addFmt(.err, v.span, "global '{s}' must be initialized by a compile-time constant; '{s}' is not a usable constant here", .{ vd.name, id.name });
+                        break :blk null;
+                    },
                     .ambiguous => {
                         if (self.diagnostics) |d|
                             d.addFmt(.err, v.span, "'{s}' is ambiguous: it is declared in multiple flat-imported modules; qualify the reference or remove the duplicate import", .{id.name});
@@ -1570,7 +1575,7 @@ pub fn selectNominalLeaf(self: *Lowering, name: []const u8, from: []const u8, ra
 pub fn isNamedTypeKind(raw: resolver_mod.RawDeclRef) bool {
     return switch (raw) {
         .struct_decl, .enum_decl, .union_decl, .error_set_decl, .protocol_decl, .foreign_class_decl => true,
-        .fn_decl, .const_decl, .namespace_decl => false,
+        .fn_decl, .const_decl, .var_decl, .namespace_decl => false,
     };
 }
 
@@ -1598,7 +1603,7 @@ pub fn namedRefTid(self: *Lowering, ref: resolver_mod.RawDeclRef, name: []const
         .enum_decl => |d| (table.type_decl_tids.get(@ptrCast(d)) orelse table.findByName(table.internString(name))),
         .union_decl => |d| (table.type_decl_tids.get(@ptrCast(d)) orelse table.findByName(table.internString(name))),
         .error_set_decl, .protocol_decl, .foreign_class_decl => table.findByName(table.internString(name)),
-        .fn_decl, .const_decl, .namespace_decl => null,
+        .fn_decl, .const_decl, .var_decl, .namespace_decl => null,
     };
 }