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/interp.zig

@@ -918,22 +918,6 @@ pub const Interpreter = struct {
             .ret_void => return .ret_nothing,
             .@"unreachable" => return error.Unreachable,
 
-            // ── Heap operations ─────────────────────────────────
-            .heap_alloc => |u| {
-                const size_val = frame.getRef(u.operand);
-                const size: usize = @intCast(size_val.asInt() orelse return error.TypeError);
-                const hp = self.heapAlloc(size);
-                return .{ .value = .{ .heap_ptr = hp } };
-            },
-            .heap_free => |u| {
-                const ptr = frame.getRef(u.operand);
-                switch (ptr) {
-                    .heap_ptr => |hp| self.heapFree(hp),
-                    else => {},
-                }
-                return .{ .value = .void_val };
-            },
-
             // ── Builtin calls ──────────────────────────────────
             .call_builtin => |bi| {
                 return self.execBuiltin(bi, frame, instruction.ty);