Add compile-time typestates and infallible accessors

[yi-json]

Foundation: Compile-time Type States and Checked Field Accessors

This PR adds optional compile-time typestate tracking (UncheckedState, CompleteState) to the experimental Rust runtime and compiler backend.

The main goal is to let callers read fields infallibly via .read() without .try_read().unwrap() or .expect() calls in consumer code once a view is validated with .check_complete(). Furthermore, it introduces a self-consuming Writer pattern (write_<field>(self, val) -> <Typename>Writer<S, NextState>) that safely tracks typestates on mutation and degrades layout-affecting fields back to UncheckedState.

All generated views default to ST: State = UncheckedState, so existing code continues to compile and work without breaking any changes

What's included in this stack:

1. rust: Implement compile-time typestates and infallible read accessors:

• Defines typestate markers (UncheckedState, CompleteState) and traits (State, IsComplete,
  CheckComplete, InfallibleRead) in typestate.rs.
• Updates the Rust code generator (emboss_codegen_rust.py and templates) to parameterize generated
  struct views by typestate (<TypeName><S, ST = UncheckedState>) and implement CheckComplete.
• Enables infallible .read() access on CompleteState views.

3. rust: Implement self-consuming Typestate Writer for safe mutations:

• Generates dedicated <TypeName>Writer<S, ST> types and .into_writer() conversions.
• Emits infallible write_<field>(self, val) setters directly on generated Writer types.
• Incorporates AST layout-dependency analysis so that writing to a field which affects downstream
  layout (e.g. dynamic offsets/sizes) degrades the returned Writer to UncheckedState, while writing to
  fixed non-layout fields preserves CompleteState.
• Adds unit and integration test suites in dynamic_size_test.rs

Testing

• cargo test in runtime/experimental/rust
• bazel test //compiler/back_end/experimental/rust/testcode/...

[jasongraffius]

LGTM, looks like some of the verification checks are failing, try reformatting the python with black.