Only refuse partial ready --fix files that formatting changes

[cgraf78]

Summary

Stop sley ready --fix --commit from blocking commits of partially
staged files whose staged content is already formatter-clean.

The commit gate refused every partially staged file (both staged and
unstaged hunks) upfront, to avoid folding formatter output across the
unstaged hunk. But that also blocked the common "stage some hunks, keep
editing, then commit" workflow even when nothing needed formatting — a
regression the dotfiles githook-test guards against ("partial-stage
clean content: commit succeeds").

Decide per file by result instead of refusing upfront:

• Pass 1 probes partially staged files without mutating them: back the
  worktree copy up, format, then always restore it. If formatting would
  have changed the file, record it; a clean partial file is a pure no-op.
  If the backup can't be made, refuse rather than risk an unrecoverable
  clobber.
• If any partial file would be reformatted, refuse before touching the
  rest of the batch, so a refusal never leaves other files reformatted in
  the worktree.
• Pass 2 formats every non-partial file in place as before, reporting the
  re-stage requirement when the formatter changes a fully staged file.

The standalone sley fix command still refuses partial files outright;
the commit gate is deliberately more permissive so clean partial-stage
commits succeed.

Testing

• update the ready --fix --commit partial-staging tests: a partial file
  that the formatter WOULD change is still refused (exit 2) with the
  worktree left intact (the probe reverts); a mixed batch refuses before
  the fully staged file is ever formatted
• add a regression test mirroring the commit hook: a clean partial-staged
  file with a no-op formatter is NOT refused, the worktree is left intact,
  and the run reaches exit 0
• verified the dotfiles githook-test passes against this build
  (PATH=<worktree>/bin → 51 passed, 0 failed), including the two
  partial-stage assertions that fail on the previous behavior
• full suite green (./test/sley-test: ok); checkrun lint and
  shellcheck -x lib/sley/ready.sh clean