Git Log: bottom-docked history tool window (JetBrains-style)

[dG9hc3Q]

Summary

Add a Git Log tool window, docked like JetBrains' "Git" tool window:
a bottom-docked panel with the commit-graph list on the left and a
selected-commit detail view on the right. It sits alongside the existing
Rhyme Search bottom panel and is toggled from the bottom tool-window stripe.

Motivation

Rhymr already has VCS baked into the editor — change bars in the gutter
(editor::vcs_gutter + git::ops::line_changes), a branch segment in the
status bar, and commit/push/pull/fetch actions (git/dialog.rs,
app.git-*). There is no way to see history: what changed, when, and by
whom. For a lyric IDE where version control is a headline feature, a
browsable log is the missing piece.

Reference — JetBrains Git Log

• Left: reverse-chronological commit list with the branch/merge graph
  rail, columns for subject, author, date; branch/tag ref labels on the
  relevant rows; a "load more" tail (paged, not the whole history at once).
• Right (or below): selected commit's metadata (full hash, author,
  committer, date, full message) and its list of changed files.
• Double-click a changed file -> diff of that file at that commit.
• Filter/search bar: by branch, by author, by message substring.

Scope the first version to the left list + commit detail + changed-file
list. Per-file diff view and the filter bar can be a fast follow.

UI / docking

• New bottom-docked panel, same mechanism as Rhyme Search: a child of the
  vertical Paned in app/layout.rs, hidden until toggled.
• app/chrome.rs::bottom_stripe currently hardcodes a single "Rhyme
  Search" toggle — generalise it to host multiple bottom tool-window
  buttons ("Rhyme Search", "Git"), or add a second stripe entry. Only one
  bottom panel visible at a time (JetBrains behaviour), or allow the
  Paned to show whichever was last toggled.
• Follow the rhyme/search.rs::RhymeSearch widget shape:
  get_widget() / is_collapsed() / set_expanded() / connect_toggle().
• Wire an app.git-log gio action + accel in app/menu.rs (JetBrains uses
  Alt+9 for the Git tool window; pick something consistent with the
  existing accel scheme), and a "Git Log" item in the Git menu.
• Styling: classic/boxy, --radius-* = 0, driven from SCSS
  (assets/scss/…, new stem registered in CSS_FILES in both
  src/css.rs and build.rs). Graph rail colours come from the theme
  palette in src/css.rs (PALETTE, (name, dark, light)), not hardcoded.
• Icons via crate::app::icons::img (there are vcs/git-* NetIcons in
  the bundled set already).

Data layer — git/ops.rs

Add history reads to GitController (git2 revwalk), keeping the existing
error-handling and threading conventions in that module:

• fn log(&self, start: LogStart, limit: usize, skip: usize) -> Result<Vec<CommitSummary>, _>
  — paged; LogStart = current HEAD / a branch / all refs.
• struct CommitSummary { id, short_id, summary, author_name, author_email, time, parent_ids } — enough to render a row and draw the graph edges.
• fn commit_detail(&self, id) -> Result<CommitDetail, _> — full message,
  author + committer, and the changed-file list (git2::Diff of the
  commit against its first parent; handle the root commit).
• fn ref_labels(&self) -> Result<HashMap<Oid, Vec<RefLabel>>, _> — local
  branches, remote branches, tags, for the ref chips.
• (fast follow) fn file_diff_at(&self, id, path) -> Result<…, _> for the
  double-click diff.

One representation for a commit — don't add a parallel model next to
whatever line_changes / file_statuses already use.

Behaviour

• All git reads run off the GTK main thread; results marshalled back to
  the UI (CLAUDE.md non-negotiable — no blocking work on the main thread).
• Load the first page (~100 commits) on first open; append pages on scroll
  to bottom / "Load more".
• Refresh on demand (toolbar button in the panel) and after any in-app
  commit/pull/fetch (hook into the same WorkspaceController refresh path
  that updates the branch label and gutter).
• Selecting a commit populates the detail pane; empty state when nothing
  selected.
• When the workspace isn't a git repo, the panel shows an inert "Not a git
  repository" placeholder (mirror how the status-bar branch segment / gutter
  handle that).
• Graph rendering: a GtkDrawingArea column (or per-row drawn cell)
  painting parent edges — start simple (single-lane for linear history,
  widen lanes for merges); it doesn't need to be pixel-perfect with
  JetBrains on day one.

Out of scope (separate follow-ups)

• Committing / amending / reverting / cherry-pick / branch checkout from
  the log (context-menu actions).
• Interactive rebase.
• Blame.
• Diff viewer polish beyond a plain per-file textual diff.

Acceptance criteria

• Bottom-docked "Git" panel, toggled from the bottom stripe + an
  app.git-log action/accel + a Git-menu entry.
• Commit list: subject / author / date, ref-label chips, paged loading.
• Commit detail pane: full hash, author, committer, date, full
  message, changed-file list.
• History reads added to GitController via git2 revwalk, one commit
  model, typed to the module's existing error style.
• All git reads off the main thread; panel refreshes after in-app
  commit/pull/fetch.
• Non-repo and empty-selection states handled.
• SCSS stem registered in css.rs + build.rs; graph/theme colours
  from the src/css.rs palette; icons via icons::img.
• cargo fmt + cargo clippy clean; syllable regression suite
  unaffected.