Draft: display dma optimization

[cofob]

What

• Added framebuffer-free retained rendering with tile-based damage tracking.
• Overlapped rendering and DMA using two alternating RGB565 buffers.
• Reduced repeated work with cached layouts, lookup tables, RLE checkpoints, and indexed commands.
• Replaced SPI with an 80 MHz PIO DMA link, cutting full-frame transfer time to 15.36 ms.
• Added flash-based background templates and stricter display-failure handling.

How it was tested

• nix develop -c ./scripts/check.sh passes locally
• On hardware: works faster :D
• Host-only change (CLI / docs / CI) — no device behavior touched

Checklist

• Device behavior / image changed → config.device_release (bcdDevice) bumped by one (hex)
  in firmware/src/main.rs — host-only (CLI / docs / CI / build) does not (CONTRIBUTING.md)
• Fixes something a downgrade would reopen → flag whether the next release should advance the
  rollback epoch (docs/production.md, stage 3) — a seal-time --rollback call, not a code bump
• New or changed unsafe → justified in docs/unsafe.md
• User-visible behavior (kinda) → the matching guide under docs/ updated
• New files carry the SPDX header (AGPL-3.0-only)
• Commit messages use the zone prefix style (fido:, piv:, rsk:, docs:, …)

[github-actions]

Development firmware for this pull request: https://rskey.fob.wtf/preview?pr=99

The link always opens the latest successful preview build.