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Typed Python §4, phase 2: the type checker (#634)

* docs: Typed Python §4 spec: static type checking before dynamic type checking

Section 3 is now "Static Type Checking" and section 4 "Dynamic Type Checking". The two references
from the static section to the dynamic one say "below".

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

* Typed Python §4, phase 2: build the checker's worker

The type checker of Typed Python §4 is pyright, in a worker (docs/typed-python-4.md). `yarn
build:typecheck` builds dist/TypedPy4Checker.js: the web worker of browser-basedpyright with the
third-party stubs and all but 59 standard library stubs removed (18 MB to 4.7 MB; 0.77 MB brotli),
and with Source Python's `builtins.pyi` and `types.pyi`, the prelude and the configuration baked
in, so that the worker is one file with everything that it needs.

* src/typecheck/bundler/: the edits of the worker's embedded typeshed (bundle.ts), the changes to
  typeshed that make Python §4's rules (source-builtins.ts: strict bool, bool is not a number, the
  types of **), and the build with a stamp, so that it is rebuilt when a source changes, and a lock,
  so that parallel callers wait for one build (typed4.ts). Each change to typeshed checks that
  typeshed has the shape that it expects, and fails with what to update if a newer one does not.
* src/typecheck/prelude.pyi: the types of the names of Python §4 that Python does not have.
* src/typecheck/config.ts: pyright's configuration. constants.ts: where programs are.
* browser-basedpyright is pinned (1.40.2) as a dev dependency; its polyfill dependencies add about
  90 packages to yarn.lock, which the worker does not need. acorn, which reads the worker's source,
  was already in the lockfile.

Tests of the build (on a small bundle, and on typeshed's shapes): src/tests/typecheck-build.test.ts.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

* Typed Python §4, phase 2: the checker

TypedPythonChecker (src/typech... (continued)

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