Add primary-branch option to preserve primary branch spine

[essteec]

Closes #181

Summary

Adds the -b, --primary-branch <BRANCH> CLI option and core.option.primary_branch configuration setting.

When specified, Serie preserves the first-parent spine of the specified branch along column 0, keeping the trunk visually straight even when feature branches contain newer commits.
When omitted, Serie preserves its existing behavior exactly.

Behavior

• Lane assignment: The primary branch spine is identified upfront and pinned to column 0 during lane assignment, placing feature branch commits in subsequent columns (column >= 1).
• Fallback & Notifications: If the specified branch cannot be resolved, or if its tip is excluded from loaded commits due to --max-count, Serie falls back to the standard layout and displays a warning via Serie's native status-line notification (StatusLine::NotificationWarn). This avoids writing to stderr while Ratatui's alternate screen buffer is active.
• Branch resolution: Resolves the specified branch name against repository local branches (Ref::Branch).

Tests & Docs

• Graph regression test: Added primary_branch_001 to src/tests/graph.rs with image snapshots covering both chrono and topo orderings under tests/graph/.
• Unit tests:
  • primary_branch_merge_back: Verifies column stability when feature branches merge back into the primary trunk.
  • primary_branch_fallback_unresolved_or_none: Verifies fallback to default layout and status warning when the branch name cannot be resolved.
  • primary_branch_max_count: Verifies fallback when tip is truncated, and visible spine pinning when older commits are truncated.
• Documentation: Updated README.md CLI help block, docs/src/getting-started/command-line-options.md, docs/src/configurations/config-file-format.md, and config.schema.json.

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[lusingander]

Thank you for the PR! It's mostly ready, but I left some comments for minor details. Could you please check them?

[essteec]

I've made find_primary_branch_tip private and switched to exact case-sensitive matching. Thank you so much for reviewing.

[lusingander]

Thank you!