feat(core): add zero-width negation expression

[floze-the-genius]

This patch was produced with assistance from an LLM-based coding agent.

Issues

Fixes #3848.

Description

Add expr::Not, a zero-width assertion that succeeds only when its inner expression does not match at the current cursor. Unlike patterns::Invert, it accepts any Expr, including anchors such as AnchorStart.

This provides the missing building block for rules that should match a phrase everywhere except the beginning of a chunk.

Demo

Not applicable; this is a core expression primitive.

How Has This Been Tested?

• CARGO_BUILD_JOBS=1 cargo test -p harper-core expr::not -- --nocapture - 2 passed
• CARGO_BUILD_JOBS=1 cargo clippy -p harper-core --lib -- -D warnings - passed
• cargo fmt --all -- --check - passed
• git diff --check - passed

AI Disclosure

• I am a human and didn't use any AI.
• I used LLM features of my editor, but not an agent.
• I used an AI agent interactively.
• I am an agent or I got an agent to do the work autonomously.

If Your PR Implements or Enhances a Linter

• I made up the sentences in the unit tests.
• The sentences in the unit tests were generated by an AI.
• I'm using examples from the bug report / feature request.
• I collected real-world sentences for the unit tests.

Checklist

• I have performed a self-review of my own code
• I have added tests to cover my changes
• I have considered splitting this into smaller pull requests.

[hippietrail]

I pasted this code into the branch I was working on when I posted #3848 and it does indeed seem to work. Thank you. I'm requesting a review from somebody much more familiar with this aspect of Harper than I am.