feat(tui): pane-ordinal badges link split panes to session-list rows

[edwin-zvs]

What

With two or more split panes open, locating a pane's session in the session list (and the reverse) meant reading and comparing titles. This PR gives each pane a 1-based ordinal in layout order, rendered as a single-digit badge (digit over a highlight background) in two places that always agree:

• Pane title — immediately before the session's status glyph.
• Session list — in the first column of the row of every session currently visible in a pane.

Design points (recorded in specs/0199-split-pane-ordinal-badges.md):

• Nothing moves. The badge replaces an existing cell: the reserved marker-gutter cell on depth-0 rows, the first rail/indent cell on nested rows, and a widened title lead-in in the pane title. Glyph/title/harness columns keep their exact positions with or without badges.
• Brightness tracks focus. The last-focused pane's badge (and its session's list badge) wears the selection colors; every other pane wears the inactive highlight.
• Single pane → no badges. A lone pane needs no locating aid.
• Positional numbering. Non-session panes (services, projects, empty) still occupy a slot so a badge always states the pane's on-screen position; group/empty pane titles wear their badge too. A session shown in several panes wears the focused pane's number when it's one of them, else the first in layout order.
• Ordinals past 9 are not rendered (never widen the badge beyond one cell).

Testing

• pane_ordinals_follow_layout_order_and_prefer_the_active_pane (app): single-pane empties, layout-order numbering, active-pane preference, slot stability when a pane empties.
• pane_ordinal_badge_replaces_the_first_cell_without_moving_columns + pane_ordinal_badge_brightness_tracks_the_active_pane (ui): width invariance at every depth, badge-first placement, marker survival on nested rows, >9 fallback, style split.
• Full workspace cargo test green.

🤖 Generated with Claude Code

[edwin-zvs]

Demo (after-only — brand-new affordance): three shell sessions, three panes.

What to look for:

• Pane titles wear 1 / 2 / 3 immediately before the status glyph, in layout order (left, right-top, right-bottom).
• The list's first column wears the matching digits; the third session (visible in no pane) has no badge and its columns don't move.
• Panes 1 and 2 show the same session — its row wears 1 (first pane in layout order), while the focused pane 3's badge and its row badge are the bright ones.

Video: pane-ordinals.mp4 (23s — splits being created, then badges appearing as the layout goes multi-pane)

[edwin-zvs]

Updated per feedback: the pane badge moved from inside the title to on top of the border, at the top-left corner (painted after the pane renders, so every pane — session, service, project, empty — wears it uniformly; all the title-offset math reverted).

Video: pane-ordinals-corner.mp4