fix(tui): align child conversation presentation with main

[tt-a1i]

Problem

Refs #681. Opening a child conversation bypasses the bound Session's public tool renderer chain, so Code Mode expands raw scripts/results while the main conversation shows OpenPI's compact call summaries. Child messages also ignore thinking visibility and Markdown presentation settings.

Value

Direct and Workflow child pages use the same compact tool presentation and native message components as the main conversation. Ctrl+O retains native evidence, including errors and streamed results.

Approach

Resolve child tool presentation through the Session's public renderer chain. Register ordinary activity-card presentation independently of execution on newer Pi hosts; reuse the same projection locally on Pi 0.99.1. Pass child thinking, output padding, code indentation and extension Markdown transformers to native message components, with settings-aware caching.

Investigation and acceptance boundaries: docs/research/CHILD_RENDER_PARITY_2026-10-05.md.

Validation

• Paired native component regression passes on Pi 0.99.1 and installed Pi 1.0.2: seven built-in tools plus Code Mode, partial/success/error events, 40/80 columns and collapsed/expanded transitions; executor identity/model surface preserved.
• Existing focused transcript, page and activity-renderer tests: 51 passed before the final regression addition.
• Independent source review and isolated native SDK smoke: no blocking findings.
• bun run check: passed; documentation contract separately rechecked after adding the investigation.
• bun run test: passed, Node 2295 passed / 9 skipped in 142.51s; UI 1158 passed in 24.52s. Final independent review passed; all 17 exact-head CI jobs passed, including required Node 22.19.0 / Node 24 / Background terminals (Windows) checks.
• Real running terminal pixels remain unverified; the installed dirty source checkout was preserved.

Impact

• User-visible: matching child tool/message presentation and inherited thinking visibility.
• Model-visible context/tools: unchanged; presentation does not replace child executors.
• Runtime/lifecycle: ephemeral renderer resolution and settings-aware UI cache; execution/cancellation semantics unchanged.
• Persisted config/data: unchanged; historical bounded previews stay previews.
• Compatibility/risk: Pi 0.99.1 fallback and Pi 1.0.2 public renderer seam verified. Main Pi's private Mermaid transformer and fullscreen image ownership are outside this public seam and remain explicit follow-ups.