fix(cli): indent stack members instead of a reserved gutter lane

[lettertwo]

This PR is not about gh-stack. It fixes a pre-existing ADR-026 bug that a Graphite user with one
tracked stack hits today.

The lane graph let a trunk hand its own lane to its primary child. With one stack and no forks,
every row rendered flush left, indistinguishable from an unrelated worktree, and the trunk carried
no marker. Indentation carried no information in the single-stack case, which is the common case.

Column 0 is now reserved for bases. A trunk or an ungrouped worktree renders flush left, and every
stack member sits in column 1 or further right, so indentation alone signals membership. A base
never hands its lane to a child, so all its children close on its row (◉─┴─╯ main) and graph width
is the number of concurrent stacks on that trunk, independent of any one stack's depth.

Ordering changes from most-active-subtree to tallest subtree first, with activity demoted to a
tiebreaker and branch name as the final tiebreak. TreeNode gains subtree_size.

docs/adr/026-graphite-lane-graph.md is rewritten to match. An earlier revision of it reserved lane
0 as a dedicated membership column, which is kept there under "Superseded".

The change is textually dependent on the gh-stack work: it touches stack_number in six places,
TreeNode::stack_number exists only because of Stack::number, and it edits lane examples in
docs/recipes/stacked-diffs.md.

Testing: 651 tests, clippy clean.

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