fix(migrate): compare owned files, not raw counts, in what_if_mutates_nothing

[aaronsb]

Summary

cmd::migrate_exec::tests::what_if_mutates_nothing fails intermittently on macOS with assertion left == right failed: what-if must not change the tree / left: 43 / right: 44.

This is a known, already-diagnosed flake — just never fixed in this one place. phase_backup carries the diagnosis in a comment:

Compare by set, not raw count: macOS Spotlight can drop a .DS_Store into a directory between the pre-count and the copy, and such OS-transients aren't part of the user's tree — counting them made the backup fail spuriously on macOS only (the 43/44 flake). If a real file is ever missing, it's named.

phase_backup was hardened with a set diff plus an is_os_transient exemption. what_if_mutates_nothing still does assert_eq!(count_files(&dest), before), so the identical Spotlight write still fails it — with the identical 43/44, and with no indication of which path appeared.

This applies the same treatment: compare the set of owned files (transients filtered out) before and after. A genuine what-if mutation now names the offending path rather than printing two integers.

Why now

Surfaced while landing #354 (ways corpus --verbose), whose macOS CI leg went red on this test. #354 touches only corpus.rs, main.rs, and a docs page — nothing that could reach migrate_exec. Confirmed it's not that PR:

• main @ portability, dispatched fresh: macOS passes
• feat(corpus): --verbose to trace phases and stream way-embed progress #354 commit 1: macOS passes
• feat(corpus): --verbose to trace phases and stream way-embed progress #354 commit 2 (a 6-line stderr change in corpus.rs): macOS fails, twice

Nondeterministic, macOS-only, and independent of the diff — consistent with Spotlight timing on the runner. Fixing it here, on main, rather than smuggling it into the diagnostics PR.

Test plan

• cargo test -p ways -- --test-threads=1 — 346 pass, 0 fail
• cargo clippy -p ways --all-targets — no new warnings (count_files still used by phase_backup, so no dead-code)
• Cross-platform CI on this branch is the real check: the macOS leg is the one that matters