fix(display): tighten commit-output file-stats footer

[CaicoLeung]

What

Two footer fixes for the landed ✓ commit output (and its confirm-mode preview, which shares emit_file_stats):

1. Suppress zero per-file counts (6c8b541) — a file with +0 or −0 rendered a noise +0/−0 column. Now the zero column renders blank, matching git diff --stat (which never shows zeros). The Σ total row keeps both totals even at zero, like git's own summary line. Column-width math is unchanged — the grid was already sized to the totals, so geometry is stable.
2. Drop the duplicate file count in the elision line (079e216) — … N more (N files) said the same number twice; now just … N more.

Deliberately not included

The exploratory spacing work from this branch (blank row between batch blocks, blank line between subject and body) was dropped after review — the landed output stays compact with no blank separation between the [n/m] row and the commit contents:

  [1/1] ✓ f67146e feat: add tracked scratch line
    +1 −1  tracked.txt
    +3     other.txt   [new]
  Σ +4 −1  (2 files)

Verification

• cargo test — 417 passed / 0 failed
• cargo fmt --all -- --check — clean
• cargo clippy --all-targets -- -D warnings — clean
• Byte-level PTY capture of the real binary confirms the landed shape above