docs/parser mismatch: README documents `union` syntax that the parser rejects

[zvndev]

Surfaced by the v0.4.2 post-release smoke test.

README §PowQL documents:
```
(User filter .age > 30) union (User filter .city = "NYC")
```

Running this against the v0.4.2 server returns:
```
Error: expected statement, got '('
```

The parser does not accept the parenthesised set-op form documented in the README. Either:

• The parser should accept it (fix the parser), or
• The README should document the form the parser DOES accept (fix the docs).

P3 — bug is documentation/parser inconsistency, not data corruption. Filed during the v0.4.3 release cycle. Not a release blocker for v0.4.3 (which fixes the P0 alter+crash bug, see #70).