feat(tui): matrix-rain horizontal words link to their session (#140)

[edwin-zvs]

Closes #140.

Matrix-rain horizontal reveal words are now mouse-interactive:

• Hover → a one-line tooltip naming the session that produced the word (<title> · <harness>, or <harness> when there's no title, or <word> · session ended if it's gone).
• Click → switches the selection to that session.

How

• Tag words with their source session. RevealWord gains a session_id, threaded through every queue path — observe_pty_activity (PTY harness words), observe_event (structured tool/status words), and observe_tool_decision.
• Record hit targets. Each frame, render_matrix_rain records every horizontal word's screen span as a MatrixRevealHit { col_start, col_end, row, text, session_id } on the App. The span covers the whole word, so it's hover/click-able even before all letters have pinned in (per the issue: "even if it is not yet completed"). Hits are cleared at the top of the render so a hidden / too-small panel can't leave stale targets clickable.
• Hover tooltip. Mouse motion is already captured (EnableMouseCapture); when the cursor is over a hit, a tooltip is drawn on the row above the word (below it at the panel's top edge), clamped to the panel width, using the selection-highlight palette so it reads over the rain.
• Click. handle_left_click resolves the hit and selects the source session (focusing the list, matching a list-row click). The matrix panel is its own region, so this is checked ahead of the pane hit-tests without shadowing real list/view clicks. A click on a word whose session has ended is a no-op with a status message.

Tests

• pty_activity_tags_reveal_with_session / observe_event_tags_reveal_with_session — words carry their session id from both the PTY and structured-event paths.
• matrix_reveal_click_switches_to_source_session — clicking a recorded hit switches the selection; clicking off the span, or on a word whose session has ended, leaves it unchanged.
• Full suite green: 133 unit + 1 integration.

Note: the tooltip/click are hard to capture in a static screenshot (reveal words appear and fade with rain activity), so this relies on the unit coverage above; happy to record a demo if useful.

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