Restructure documentation to match template style

- Add Ruby API, C API, Hardware Configuration sections to dvi.md
- Extract text mode rendering to dvi/text-mode-rendering.md
- Extract stability analysis to dvi/stability.md
- Update dvi.md architecture section hierarchy and layout

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: harukasan

CLAUDE.md

@@ -24,6 +24,8 @@ Design documents and implementation notes are in `doc/`:
 - [doc/psram.md](doc/psram.md) — PSRAM driver (APS6404L, QMI CS1 initialization, XIP mapping)
 - [doc/dvi.md](doc/dvi.md) — DVI output (HSTX, DMA, text/pixel modes, stability analysis)
 - [doc/dvi/batch-rendering.md](doc/dvi/batch-rendering.md) — Batch scanline rendering (N=4, line buffers, descriptor layout)
+- [doc/dvi/text-mode-rendering.md](doc/dvi/text-mode-rendering.md) — Text mode rendering (VRAM, font cache, scanline renderer)
+- [doc/dvi/stability.md](doc/dvi/stability.md) — DVI stability analysis (bus contention, flash write safety, diagnostics)
 - [doc/usb-host-keyboard.md](doc/usb-host-keyboard.md) — USB host keyboard (PIO-USB, HID, Ruby/C API)
 - [doc/filesystem.md](doc/filesystem.md) — FAT filesystem on flash (FatFs, VFS, flash write safety)