Implements read-write (`+r` / `+{reg}`) `-> @place` outputs. LLVM has
no `+` constraint, so a read-write place lowers to:
- an output `=` constraint (return slot, stored back through the
place after the call), with the leading `+` rewritten to `=`; plus
- a TIED input constraint (the decimal index of that output) appended
after the regular inputs, seeded with the place's loaded value
passed as a call arg.
Tied inputs are appended last so existing operand indices (%[name] ->
${N}) are undisturbed; asmOperandIndex stays correct. Lowering no longer
rejects `+` (indirect `*` still rejected). emitInlineAsm grows the
arg/param arrays by the rw count, loads each seed, and emits the tied
constraint.
Verified by running: increment-in-place (41 -> 42) and a mixed case
(rw place + regular input + value output) producing the textbook
"=r,=r,r,0" constraint with correct ${N} indices. 1650 flipped from
the rejection lock to a runnable aarch64-pinned example (ir-only
elsewhere). zig build test green (658 corpus, 446 unit).
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).
