Cache listTitleWidth result to avoid per-frame O(N×V) lipgloss.Width recomputation

[gadflysu]

Problem

listTitleWidth() is called once per visible row inside renderRowFull(), which is called for every row in every View() invocation. In stateList mode the method iterates over all sessions twice — once for titles, once for CWD strings — calling lipgloss.Width(display.Sanitize(...)) on each to find the natural maximum widths:

// picker/model.go  listTitleWidth()
for _, s := range m.sessions {
    if w := lipgloss.Width(display.Sanitize(s.Title)); w > maxNaturalTitle { ... }
}
for _, s := range m.sessions {
    if w := lipgloss.Width(display.Sanitize(s.CWDDisplay)); w > maxDirW { ... }
}

lipgloss.Width calls ansi.stringWidth → FirstGraphemeCluster → uax29/graphemes for Unicode grapheme segmentation — not a trivial operation.

With V visible rows and N total sessions, each frame triggers V × 2N width computations. For V=30, N=200 that is 12,000 calls per View(), repeated every 120 ms tick.

Evidence

CPU profiling via sample(1) on a live aps process shows the hot path:

bubbletea.eventLoop (120 ms tick)
  → View() → renderList() → renderRowFull() ×V
      → listTitleWidth()
          → lipgloss.Width(Sanitize(title))  ×N
          → lipgloss.Width(Sanitize(cwd))    ×N
              → ansi.stringWidth → FirstGraphemeCluster
                → uax29/graphemes Unicode grapheme split   ← CPU hot spot

listTitleWidth accounted for the majority of user-space samples in the profile. By contrast, idColW and msgColW are already computed once in newModel and never recalculated.

Fix

Add two cached fields to Model:

naturalTitleW int  // max lipgloss.Width(Sanitize(s.Title))   across m.sessions
naturalDirW   int  // max lipgloss.Width(Sanitize(s.CWDDisplay)) across m.sessions

Compute them in a helper recomputeNaturalWidths() and call it from:

• newModel (initial)
• applyRefresh (sessions added or updated)

listTitleWidth() reads the cached values directly — no per-call iteration.

Non-goals

• Does not change visible layout or column widths
• Does not affect stateListPreview path (already skips the natural-width bonus)

Plan

docs/agent/plan-issue-49-cache-list-title-width.md