feat(parser): accept parenthesised singleton def receiver

[tannevaled]

Gap

MRI's `def` grammar allows the singleton receiver to be a parenthesised arbitrary expression: `def (expr).method(...) ... end`. The parser only accepted an unparenthesised name (`def self.m`, `def Const.m`, `def obj.m`, ivar/cvar/gvar), rejecting the parenthesised form with expected method name after def.

```ruby
obj = Object.new
def (obj).kw(a:); a; end # MRI defines singleton kw on obj; parser errored
def (String).foo; "hi"; end # parenthesised const receiver
```

Change

Add the `def ( ) . ` production. A `(` right after `def` can only open this form (a method name is never a paren group), so it is parsed as the receiver: a single expression (local, constant, method call, `self`, ternary, `and`/`or`, assignment) evaluating to the receiver object, exactly as MRI. A compound group (`def (a; b).m`) is a syntax error in MRI and is rejected here too.

Same AST as the bare form

The production emits the identical `MethodDef` shape as the bare `def obj.m` form — `Singleton=false` with the parsed receiver in `Recv` — so the compiler lowers it with no special case (rbgo `compileSingletonReceiver` already handles an arbitrary-expression receiver). When the receiver is a known local or a constant, the node is byte-identical to the bare form's; a table test asserts this.

Verification

• MRI `ruby` 4.0.x matched on paren-local, paren-const, paren-method-call, `(self)`, endless `= expr`, and compound-receiver rejection.
• Tests: paren-local/const/method-call/self receivers, positional/keyword/paren-less params, endless body, compound rejection.
• 100% statement coverage; `go vet` clean; builds on 6 64-bit arches + js/wasm + wasip1/wasm.