fix(git): preserve log stat summary lines

[arasovic]

Summary

• Preserve git log --stat summary lines when RTK's injected log format places stat rows after ---END---.
• Keep stat rows attached to the correct commit and make the omission marker count file rows only.
• Scope is intentionally limited to the git log --stat drops summary line on 4+ file commits; omit marker conflates file rows with metadata #1704 correctness bug; it does not attempt the broader stat compression work discussed in git log --stat output passes through uncompressed #611 / fix(git): compress --stat histogram bars in git log output #698.

Fixes #1704.
Related: #611.
Prior art: #698.

Test plan

• cargo fmt --all --check
• cargo clippy --all-targets
• cargo test filter_log_output - 15 passed
• cargo test cmds::git::git::tests:: - 84 passed, 3 ignored
• cargo test --all in a clean CI-like env - 1984 passed, 7 ignored
• bash scripts/check-test-presence.sh origin/develop
• Manual testing: target/debug/rtk git log -3 --stat on the git log --stat drops summary line on 4+ file commits; omit marker conflates file rows with metadata #1704 3/4/10-file repro

Manual output now keeps the summary visible for both boundary and truncated stat blocks:

3c84db2 10 files (9 minutes ago) <Repro>
  y1.txt  | 1 +
  y10.txt | 1 +
  y2.txt  | 1 +
  y3.txt  | 1 +
  [+6 file rows omitted]
  10 files changed, 10 insertions(+)
8f4835b 4 files (9 minutes ago) <Repro>
  w1.txt | 1 +
  w2.txt | 1 +
  w3.txt | 1 +
  w4.txt | 1 +
  4 files changed, 4 insertions(+)

Snapshot tests are not applicable; this change does not touch insta snapshots.
Token-savings validation is not applicable as a standalone gate because this is a correctness fix for an existing filter path, not a new filter.

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