refactor(tui): extract shared session rendering primitives

[jlongster]

Summary

First small code increment toward shared, nested session rendering.

• Move the existing session render context and hook into render-context.tsx.
• Move text/reasoning components and the shared inline tool row into message-parts.tsx.
• Preserve component bodies, context ownership, reactive reads and the existing InlineToolRow export.

This is a mechanical extraction. Grouping, scrolling, navigation, disclosure defaults and presentation remain unchanged. Larger tool-specific and group renderer extractions can follow in separate increments. The planning document and diagnostic scripts remain local.

Verification

• bun typecheck in packages/tui: passed
• Full TUI suite: 1352 passed, 4 skipped, 0 failed; 2 snapshots, 107855 assertions
• Local baseline comparison: all 67 original function bodies unchanged apart from whitespace/export modifiers; context declaration and icon width preserved
• OpenCode Drive against this checkout with --dev: replayed the baseline fixture and inspected busy exploration plus completed thought/read/text at 112x34 and 80x24. Layouts match visually; real durations and animation frames vary.
• New modules pass Prettier; staged diff passes whitespace checks

Baseline: 8f4d7066473ea07d26c5dfc35e46cd9a94e3e292. Replaces the closed documentation-only PR #48392 with an actual code increment.