feat(log): add ref decoration and HEAD marking to the commit log

[whaclaw-bot]

Closes #9.

Brings the orientation git log --decorate gives to the commit log view.

• Ref decoration chips — src/git/diff/refs.rs walks repo.references() once into an Oid -> Vec<RefLabel> map, distinguishing HEAD / local branch / tag / remote branch by color (git's own --decorate palette). Annotated tags peel to the commit they point at. Cost scales with the number of refs, not commits.
• HEAD emphasis and merge glyph — two fixed glyph cells per row: divergence (↑ ahead, v behind) and shape (* HEAD, Y merge), so rows stay column-aligned.
• Accurate ahead/behind — the old i < ahead_count positional assumption is gone. revwalk.push(local) + hide(upstream) (and the reverse) build oid sets tested by membership, so the markers stay correct under pagination and filtering. Each side is capped at MAX_DIVERGENCE_OIDS; the walk is newest-first, so the cap drops the tail no one scrolls to.
• Width-threshold wide layout — at area.width >= MIN_DETAIL_WIDTH the row switches to absolute time, author plus email, 10-char short id, and untruncated chips. Keyed on width rather than list_fullscreen, matching the diff_viewer::MIN_SPLIT_WIDTH precedent. One commit still equals one row; the extra room grows columns.
• Refresh gating — refs move without HEAD moving (a fetch advances origin/dev), so RepoSnapshot carries a refs fingerprint and the map is rebuilt only when it changes. A failed rebuild keeps the previous map rather than dropping chips on a transient error.

commit_list.rs was split into commit_list/{mod,row}.rs first, as a pure refactor, to stay under the 300-line limit.

wall_clock gained local_date_time for the absolute column — hand-rolled civil-from-days, consistent with that module's existing no-date-crate decision.

The commit graph stays out of scope, as the issue proposed: a lane graph presumes topological order, and changing the revwalk sort drags the anchor+skip pagination contract along with it. docs/architecture.md records that.

Verification

cargo fmt --check, cargo clippy --all-targets --all-features -- -D warnings, and cargo test (1435 passed, 0 failed) all pass on the merge with upstream/dev.