yeast: Support optional guards in rules

[tausbn]

Permits the use of guards of the form where expr (with expr being any Rust expression that evaluates to a boolean value) inside of rules. The guard should come after the query itself, and before the => that separates the query from the body, like so:

(foo bar: _? @bar)
where bar.is_some()
=> (baz bar: {bar})

If the guard is absent, it behaves as if it were where true. Inside of the guard body, all captures are treated as if they are raw. Translation of non-raw captures only happens if the guard succeeds.

The guard can also access the user-defined context. This context is shared with the body if the guard succeeds, and discarded if the guard fails (just as it is currently discarded after running a rule body). (I think it's unlikely that we'll ever want to mutate the context from inside the guard, but you never know...)

(NB: In a previous version, I used if instead of where but I think I like the latter better. It's a bit more prominent, and it's less likely to overlap with actual language keywords. I'll happily change it again, though.)

[Copilot]

Pull request overview

Adds optional Rust-expression guards to Yeast rules, evaluated before capture translation.

Changes:

• Adds runtime guard evaluation and context handling.
• Extends macros with guard parsing and raw capture bindings.
• Adds documentation and tests for guarded rules.

Show a summary per file

File	Description
shared/yeast/src/lib.rs	Adds guard runtime support.
shared/yeast-macros/src/parse.rs	Parses and generates guarded rules.
shared/yeast-macros/src/lib.rs	Documents macro guard syntax.
shared/yeast/doc/yeast.md	Documents guard behavior.
shared/yeast/tests/test.rs	Tests guards, captures, context, and macros.

Review details

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• Files reviewed: 5/5 changed files
• Comments generated: 2
• Review effort level: Balanced

[asgerf]

LGTM.

The code was too dense for me to have much confidence in my own ability to notice any issues with it. But I've played around with it for a while today and so far it seems to work perfectly.