feat(stdlib): expose Python type references and dict/list constructors in Builtins

[dbrattli]

Closes #290.

Summary

Fable.Python.Builtins exposes int, float, str, bytes, len, isinstance, etc. as methods on IExports, but the type values themselves (the int / dict / list Python objects) and the dict() / list() constructors weren't exposed. Downstream libraries like Thoth.Json.Python and Fable.TypedJson currently redeclare them privately as [<Emit>] boilerplate.

Changes

Type references (top-level let bindings in Builtins)

For use with Fable.Core.PyInterop.pyInstanceof:

• pyInt, pyFloat, pyBool, pyStr, pyBytes
• pyList, pyDict, pyTuple, pySet
• pyNone — Python's None singleton, typed as obj

The py prefix is necessary because int, float, bool, str, list, dict, etc. would shadow F#/.NET built-in types.

New constructors on Builtins.IExports

• bool: obj -> bool — completing the set alongside int/float/str
• dict: unit -> Dictionary<string, obj>
• dict: IEnumerable<string * 'V> -> Dictionary<string, 'V> — generic in 'V
• list: IEnumerable<'T> -> ResizeArray<'T>

Dictionary<string,'V> and ResizeArray<'T> are used (rather than obj) to stay idiomatic per BINDINGS_GUIDE.md — these are the F# types that Fable maps to Python's native dict and list.

Result

Downstream consumers can now drop the private [<Emit>] boilerplate and write:

open Fable.Core.PyInterop
open Fable.Python.Builtins

pyInstanceof v pyInt        // was: [<Emit("isinstance($0, int)")>]
pyInstanceof v pyDict       // was: [<Emit("isinstance($0, dict)")>]
builtins.dict ()            // was: [<Emit("dict()")>]
builtins.list items         // was: [<Emit("list($0)")>]
pyNone                      // was: [<Emit("None")>]

Test plan

• Added 6 new test cases in test/TestBuiltins.fs covering bool, dict() empty, dict from pairs, list, pyInstanceof against every type reference, and pyNone
• All 539 tests pass (just test-python)
• Verified generated Python is isinstance(v, int), builtins.dict(), None, etc. — exactly what the issue requested
• No breaking changes — purely additive

🤖 Generated with Claude Code