perf: build queries up to 2.7x faster by reusing ones already built (#40)

## Summary

Ports upstream 1.11.3's query interning to the builder. When a step
repeats one taken before, the builder returns the query it built then
instead of building it again.

```text
query.where(field, op, value)
  key = (step, every argument, type-tagged)       # 1 / 1.0 / True / "1" stay apart, floats by hex
  if parent._children[key] is alive: return it    # children held weakly, parent strongly
  build the child, remember it under key
  hold it in _RECENT (last 4096 built)            # CPython frees a dropped chain at once; V8 waits for GC
```

Subqueries (`related`, `exists`) are keyed by the identity of their
query's AST. The new `CorrelatedSubquery.source` field keeps that query
alive. A two-hop `exists` builds its junction AST new every time, so
it's keyed by its junction condition instead. Python's keys are exact
where upstream's aren't: upstream folds a `flip` of `'yes'` into `true`.

Stacked on #39, which this wins back the builder slowdown for.

## Evidence

Wall-clock, Python median, same machine:

| case | `main` (1.10.0) | #39 | this PR |
| --- | --- | --- | --- |
| build.simple | 3.5 µs | 4.6 µs | 1.3 µs |
| build.complex | 20.8 µs | 25.2 µs | 13.7 µs |
| query.simple.b100 | 834 µs | 907 µs | 547 µs |
| query.complex.b100 | 3.69 ms | 4.17 ms | 2.78 ms |

- `test_rebuilt_queries_match_upstream`: the same programs three times
in one batch through one builder, compared with the npm package. They
cover every step kind: scalar and float `where`, `whereFn` with a static
parameter and `exists`, two-hop `whereExists`, a raw condition,
`related` with and without a callback, `orderBy`, nested `start`,
`limit` and `one`.
- **Before** the two-hop key fix (0d4781b): fails. `scalar: true` then
`false` returned the first query.
  - **After:** passes.
- `uv run pytest`: 784 passed, 100% coverage. `uv run python -m
tests.compat`: fully exercised.

## Merge Danger

**Door:** two-way

**Blast Radius:** builder

Queries built from the same builder now share objects. Their ASTs equal
what fresh building gives, including when a caller later mutates a list
it passed in, which upstream does the same way. Memory is bounded:
children are held weakly, plus at most 4096 recently built queries.

Reviewed with sol-loop: 2 rounds with gpt-6.1-sol. Sol verdict: approved

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: jnsdls

docs/compat.md

@@ -70,7 +70,7 @@ These are the cases where the proof holds Python to something narrower than "ide
 - **Interned queries.** Upstream's builder hands back the query it built before when a step repeats one,
   keyed so that some values JavaScript's types rule out share a key: a `flip` of `'yes'` with `true`, any
   `orderBy` direction but `'asc'` with `'desc'`. In one process, such a value can come back as whichever
-  was built first. Python builds what it is given. The harness gives each program a fresh schema, which
+  was built first. Python interns as well, but by exact keys, so it builds what it is given. The harness gives each program a fresh schema, which
   upstream interns by, so that no comparison depends on the programs before it.
 - **A mutator returning `null`.** Upstream records a mutator's return value as result data unless it is
   `undefined`, so a TypeScript mutator can return `null` as data. Python has only `None`, which the port

src/zero_server/protocol/ast.py

@@ -11,7 +11,7 @@
 
 import functools
 from collections.abc import Callable, Sequence
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 from typing import TYPE_CHECKING, Any, ClassVar, Literal, Protocol
 
 from zero_server.protocol.wire import dump_json, json_model, parse_wire
@@ -192,6 +192,9 @@ class CorrelatedSubquery:
     hidden: bool | None = None
     subquery: AST
     system: System | None = None
+    # The builder query `subquery` is the AST of, kept alive with it: the builder interns a query by
+    # the identity of its subqueries' ASTs, so they must outlive the query keyed by them.
+    source: Any = field(default=None, repr=False, compare=False)
 
     def to_json(self) -> dict[str, Any]:
         out: dict[str, Any] = {"correlation": self.correlation.to_json()}