… idempotent (#22401)

## Why

The merge pass for consecutive `if (..) return X;` statements decides whether two jump bodies are equal via `content_eq`, and `ObjectProperty::content_eq` includes the `shorthand` flag. Codegen, in contrast, always prints `{ x: x }` as `{ x }` whenever the value is an identifier matching the key. The flag disagreement made the merge non-idempotent. Given:

```js
function _(body) {
  if (a) return { body };
  if (b) return { body };
  if (c) return { body };
  if (d) return { body: body };
}
```

pass 1 merged only the first three (their `shorthand` matched) and left the trailing if alone:

```js
function _(body) {
  if (a || b || c) return { body };
  if (d) return { body };
}
```

pass 2 then merged all four, because the printed `{ body }` reparses with `shorthand = true`. The fixed-point loop converges, but a second call to `Compressor::build` would still mutate the AST — the contract `monitor-oxc` checks.

## Fix

Extend `substitute_object_property` (already called once per `ObjectProperty` exit in the peephole loop, so no new traversal dispatch) to set `shorthand = true` whenever the value is an identifier with the same name as a non-computed, non-method, non-`__proto__` init key. The AST now matches what codegen would emit, so subsequent `content_eq`-based merges converge in a single pass. Checks are ordered to bail on the common case first (already shorthand, or non-identifier value); output text and the `minsize` / `allocs` snapshots are unchanged.

Originally surfaced by `monitor-oxc` CI on `@typespec/ts-http-runtime`'s `sendRequest.js`: https://github.com/oxc-project/monitor-oxc/actions/runs/25841541741/job/75927903765