Implement v2 Pratt precedence and canonical expressions

## Parent

- #7

## What to build

Deliver the accepted v2 precedence contract from pest tokens through a Pratt parser, checked typed evaluation, canonical rendering, Python results, CLI output, and normative conformance.

## Frozen contract

- `**` is right associative and binds before unary signs.
- Unary signs bind before multiplication/division, which bind before addition/subtraction.
- Integer division truncates toward zero.
- Every scalar operator returns Scalar while operand source nodes remain in the trace.
- Negative or oversized exponents fail; `0 ** 0` is `1`.

## Acceptance criteria

- [ ] RFC-0001 arithmetic case IDs pass in Rust and installed Python.
- [ ] Checked i64 arithmetic reports overflow, invalid exponent, and divide-by-zero without panic.
- [ ] Canonical expressions preserve the evaluated tree and option binding.
- [ ] V1 flat precedence and `^` exponent produce compatibility behavior or spanned migration diagnostics.
- [ ] Parenthesized pipelines target the compound result without changing parse precedence.
- [ ] Rust, Python, CLI, and documentation agree.

## Blocked by

- #7
- #17
- #39

Start with the parent and blocking issues (#7, #17, and #39), then trace pest tokens through the Pratt parser and checked evaluation. Run the RFC-0001 arithmetic cases in Rust and installed Python, and compare canonical expressions, CLI output, and documentation against the frozen precedence and error-handling contract.

Written by the indexing model from the issue text.