perf(tui): cache padding string in session list render loop

[jongio]

Summary

The session list render loop allocates new padding strings on every frame via strings.Repeat, creating thousands of short-lived allocations per second in the TUI hot path.

Problem

In sessionlist.go (lines 614-625), the Render() method pads the viewport to full height:

// Pad to full height.
for len(lines) < s.height {
    lines = append(lines, strings.Repeat(" ", s.width))
}
// ... after join + split safety net:
for len(all) < s.height {
    all = append(all, strings.Repeat(" ", s.width))
}

Each strings.Repeat(" ", s.width) allocates a new string. With a typical terminal (200 columns, 50 rows), this creates up to 50+ allocations per frame. Bubble Tea can render at 60 FPS during interaction, producing ~3,000 allocations/sec from padding alone.

Recommended Fix

Cache the padding string and invalidate only when terminal width changes:

type SessionList struct {
    // ...
    cachedPad      string
    cachedPadWidth int
}

func (s *SessionList) pad() string {
    if s.cachedPadWidth != s.width {
        s.cachedPad = strings.Repeat(" ", s.width)
        s.cachedPadWidth = s.width
    }
    return s.cachedPad
}

Then replace both call sites:

lines = append(lines, s.pad())

Impact

• Severity: Medium-High
• Affected operations: Every TUI frame render
• Expected improvement: Eliminates ~3,000 string allocations/sec during active interaction; reduces GC pressure

[jongio]

Consolidated into a single tracking issue.