Feature: in-file text/code redaction in fixture3 reduce (blocked on guardrail3 walker extraction)

[tartakovsky]

What

Add text redaction within files (in addition to the existing folder and file redaction in fixture3 reduce). Currently the reducer can remove whole files and directory subtrees; it cannot remove sections of code within a file.

Why

Real fixture trees often have files where only some functions, types, or code blocks are actually exercised by the suite. Keeping the whole file when only 20% is reachable inflates fixtures and obscures what's actually under test. In-file redaction would let fixture3 reduce produce a truly minimal fixture: not just "fewest files" but "smallest possible content per file."

Blocker

This work depends on extracting the code-walking components from guardrail3 (syn for Rust, tree-sitter for TypeScript and others) into a reusable package. Today those walkers live inside guardrail3's source-scan crates and aren't published as a standalone dependency. The extraction work belongs in guardrail3, not here.

Tracking blocker as a sibling issue when one is opened in guardrail3.

Approach (after blocker resolves)

1. Add a --reducers code flag to fixture3 reduce.
2. For each file in the trial tree, walk the AST and try removing one definition at a time (function, struct, impl, module, top-level statement).
3. Run the suite. If approved output stays stable, keep the removal.
4. Continue until no further definition can be removed.
5. Same DDMin oracle-budget model as the existing reducers.

Done

• fixture3 reduce --reducers code works on at least Rust and TypeScript.
• Documented in wiki Workflow and Commands.
• Self-suite + fake-project verifications still pass after the change.