[v0.4] The tracked tree holds nothing the repository walkers skip (CACHEDIR.TAG, target, .lexlean, .lake) (follow-up to #156) #170

Follow-up to #156 (review comment of 2026-10-17, #156 (comment)). support::holds_cache_tag (crates/conformance/src/support.rs:33-35) skips any directory holding a file named CACHEDIR.TAG. The name-based skips (target, .git, .lexlean, .lake, expected) skip by directory name. Both serve build output beside the sources, but a committed CACHEDIR.TAG, a committed .lexlean directory outside examples/, tests/ and compiler/ (not ignored there), or a force-added target/ takes a directory out of RT-01, RT-04's generated-module check, VR-19, assert_library_has_no_handwritten_lean, the authoring gates and the oracle tree digest. Nothing checks the tracked tree for that. No such path is tracked today.

Definition of done:

• A conformance case, or an extension of an existing RP row with its statement amended, reads the tracked tree (git ls-files, or the release manifest's file list) and fails when any tracked path is named CACHEDIR.TAG, or has a component the walkers skip by name, outside the paths that are legitimately committed under such names (list them, for example expected/ oracles).
• Planted defects recorded in VERIFICATION.md: a tracked examples//CACHEDIR.TAG, and a tracked .lexlean directory outside the ignored roots, each failing the case.
• The CHANGELOG line "every repository walker" is corrected, or the remaining name-only walkers (xtask/src/audit.rs:10, crates/conformance/src/lib.rs:197, cases/oracle.rs:2403) gain the same skip.
• R3 row if a new capability is claimed, just vv green, no weaker gate.