test(asm): lock read-write + place-output rejection (Phase G prep)

Adds examples/1650-platform-asm-rw-place.sx exercising a `+r -> @x`
read-write place output. Currently rejected loudly at lowering
("not yet implemented"); this locks that behavior as a passing test.
The next commit implements read-write outputs and flips this example
to run end-to-end (increment-in-place → 42).

examples/1650-platform-asm-rw-place.sx

@@ -0,0 +1,11 @@
+// ASM stream Phase 2 — read-write (`+`) place output. The place is LOADED as a
+// seed, the asm both reads and writes the operand register (tied input ↔ output),
+// and the (modified) result is STORED back through the place. Increment-in-place:
+// the register holds 41 on entry, the asm adds 1, 42 is written back to `x`.
+// aarch64-pinned; ir-only elsewhere.
+compute :: () -> i64 {
+    x : i64 = 41;
+    asm volatile { "add %[v], %[v], #1", [v] "+r" -> @x };
+    return x;   // 42
+}
+main :: () -> i64 { return compute(); }

examples/expected/1650-platform-asm-rw-place.exit

@@ -0,0 +1 @@
+1

examples/expected/1650-platform-asm-rw-place.stderr

@@ -0,0 +1,5 @@
+error: read-write (`+`) asm outputs are not yet implemented; use a write-only `=` output
+  --> examples/1650-platform-asm-rw-place.sx:8:5
+   |
+ 8 |     asm volatile { "add %[v], %[v], #1", [v] "+r" -> @x };
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

examples/expected/1650-platform-asm-rw-place.stdout

@@ -0,0 +1 @@
+