Expand query DSL support for null checks, set ops, and distinct-style queries

[elcritch]

Summary

This PR expands the query DSL with a few common SQL features and tightens the
test/docs coverage around them.

Added

• NULL support in expressions
  • nil / null literals
  • == nil -> IS NULL
  • != nil -> IS NOT NULL
• exists(...) / not exists(...)
• query-level DISTINCT
  • select \distinct` Post(author)`
  • existing count(distinct author) support remains
• query-level UNIQUE as a synonym for distinct
  • select unique Post(author)
• set operations
  • call form still works: union(...), intersect(...), `except`(...)
  • infix form now also works:

    query:
      select person(id) where id <= 2
      union
      select person(id) where id >= 4

Docs / cleanup

• removed stale completed TODOs from README.md

• updated README examples to reflect the current DSL forms

• added/updated regression tests for the new syntax

• mixed infix set-op chains like a union b intersect c are still rejected

• complex join variants (leftjoin, rightjoin, fulljoin) are still deferred

[elcritch]

The parser changes were pretty simple for these. Nice!