mem: delete .heap_alloc/.heap_free IR ops + the silent libc-malloc escape

allocViaContext used to fall back to a direct `.heap_alloc` (libc
malloc) when `Context` wasn't registered — i.e. when the program
didn't import std.sx. That was a silent escape hatch: a program could
appear to allocate fine without a `Context`, sidestepping protocol
dispatch entirely. Same shape as the matchContextAllocCall trap we
removed, just in a different code path.

Now: every site that needs `Context` emits a clear diagnostic when
the type isn't in scope, pointing the user at the required import.

- `allocViaContext`: the three fallback branches (no implicit_ctx, no
  Context type, malformed Context struct) all call the new
  `diagnoseMissingContext("heap allocation")` and return a
  placeholder. Codegen no longer emits libc malloc as the silent
  no-import path.
- `lowerPush`: the no-Context branches used to silently drop the
  push and just lower the body. Now diagnose first, then lower
  (keeping the body's other diagnostics flowing).
- `lowerIdentifier` for "context": used to silently fall through to
  `global_names.get("context")` (which would emit an unresolved
  identifier with no actionable hint). Now diagnose with the
  required-import message.

With every consumer gone, the `.heap_alloc` and `.heap_free` IR ops
are deleted entirely:

- `inst.zig`: drop the Op variants.
- `interp.zig`: drop the execInst arms.
- `emit_llvm.zig`: drop the arms (the `getOrDeclareMalloc/Free`
  helpers stay — they're still used by the foreign-decl path for
  user-level `malloc`/`free` foreign bindings).
- `print.zig`: drop the printers + the isVoidOp arm.
- `emit_llvm.test.zig`: drop the unit test (op no longer exists).

155/155 example tests pass. Unit tests green. Chess green on macOS /
iOS sim / Android. A program that doesn't import std.sx and tries to
use `context.allocator.alloc` or `push Context.{}` or the `context`
identifier now gets a real error:

  error: heap allocation requires the Context type — add
  `#import "modules/std.sx";` (or a module that imports it)

Closes the last silent allocation-protocol escape.

src/ir/print.zig

@@ -228,12 +228,6 @@ fn printInst(instruction: *const Inst, ref_idx: u32, tt: *const TypeTable, write
             try writer.print("store %{d}, %{d}\n", .{ s.ptr.index(), s.val.index() });
             return;
         },
-        .heap_alloc => |u| try writer.print("heap_alloc %{d} : ", .{u.operand.index()}),
-        .heap_free => |u| {
-            try writer.print("heap_free %{d}\n", .{u.operand.index()});
-            return;
-        },
-
         // ── Struct ops ──────────────────────────────────────────
         .struct_init => |agg| {
             try writer.writeAll("struct_init [");
@@ -522,7 +516,7 @@ fn writeConstant(val: ConstantValue, writer: Writer) !void {
 
 fn isVoidOp(op: Op) bool {
     return switch (op) {
-        .store, .heap_free, .global_set, .br, .cond_br, .switch_br, .ret, .ret_void, .@"unreachable" => true,
+        .store, .global_set, .br, .cond_br, .switch_br, .ret, .ret_void, .@"unreachable" => true,
         else => false,
     };
 }