Fix TUI panic on narrow terminal: negative strings.Repeat count

[phiat]

Summary

• Fixes a panic (strings: negative Repeat count) that crashed the TUI when the terminal was resized so that the stream pane's effective inner width dropped below 4 columns.
• Adds a regression test exercising every render branch with widths down to -10.

Root cause

StreamView.SetSize clamped innerWidth/innerHeight for the viewport but stored the raw caller-provided width in s.width. With the tree visible, model.go:456 calls SetSize(m.width-m.treeWidth-5, ...), which can go negative on narrow terminals. updateContent then computed contentWidth := s.width - 4 (more negative), passed it to renderItem, where strings.Repeat(\"─\", min(width, 60)) panicked.

Trace from the user's crash:

strings.Repeat({0x5d92cb?, 0x400?}, 0x0?)  → panic
internal/tui.(*StreamView).renderItem  stream.go:342
internal/tui.(*StreamView).updateContent stream.go:206
internal/tui.(*StreamView).SetSize       stream.go:82
internal/tui.(*Model).updateLayout       model.go:456

Fix

• Clamp contentWidth to a minimum of 1 in updateContent, mirroring the existing innerWidth clamp in SetSize.
• Defensively guard the strings.Repeat site in renderItem so a negative width can never reach it again.

Audit

I scanned the TUI for the same class of bug:

• tree.go — three strings.Repeat sites; each already gated by if X < 1 { X = 1 } or if gap >= 1. Safe.
• stream.go::truncateContent — already guards with width > 0. Safe.
• model.go::wrappedRows — guards m.width <= 0. Safe.

The only similar-class bug was the one fixed here.

Test plan

• go build ./...
• go test ./... passes
• New TestStreamView_NarrowResizeDoesNotPanic panics on the unfixed code (verified by temporarily reverting) and passes with the fix
• Manual: resize terminal narrow with tree visible — TUI no longer crashes

Closes claude-esp-5k0.

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