fix(tui): preserve Code Mode row background after truncation

[tt-a1i]

Problem

Refs #683. Truncating a Code Mode Bash preview clears the row background after the command, exposing terminal-colored ellipsis and right padding.

Value

Main and child Code Mode cards retain an uninterrupted background, including long or wrapped evidence.

Approach

Reapply the owning background after Pi's full SGR reset, preserving the reset's foreground/style semantics.

Summary

 framed(row)
-  background(row + padding)
+  background(each segment separated by native SGR reset)
+  retain resets between segments

Evidence

• Before: new ANSI-state regression fails on an unpainted ellipsis/trailing cell at 40 columns.
• After: 21/21 focused renderer/native tests pass on Pi 0.99.1 and installed Pi 1.0.2. Regression checks every visible character, including padding, at 40/120/240 columns in collapsed/expanded views with wide characters.
• Independent review: no blockers.
• Investigation: docs/research/CODEMODE_TUI_PRESENTATION_2026-10-04.md, dated background-continuity appendix.

Validation

bun run check and the documentation contract passed. One full bun run test passed: Node 2296 passed / 9 skipped in 326.49s, UI 1158 passed in 32.79s. All 17 exact-head CI jobs passed, including the three required merge checks. Installed dirty source was preserved; current running terminal pixels after reload remain unverified.

Impact

User-visible: continuous card background. Model-visible tools/results, execution/lifecycle, persisted configuration and truncation budgets: unchanged. Uses the existing public Theme background API; no host modification.

Merge Danger

Door: two-way. Small presentation-only change, reversible without data migration.

Blast Radius: CodeMode.