feat: phase 4 — Hextile decoder and recording example

codec/hextile.rs:
- Full Hextile (encoding 5) decoder per ByteColorRFBRenderer.int()
- Handles: Raw tiles, BackgroundSpecified, ForegroundSpecified,
  AnySubrects, SubrectsColoured flags
- Background/foreground colors persist across tiles
- 4 unit tests covering all subencoding paths

framebuffer.rs:
- Added fill_rect() for Hextile background/subrect fills

session.rs:
- Wired Hextile encoding 5 into the rect dispatch

examples/record.rs:
- 30-second (configurable) recording session
- Saves 1 PNG per second to out/ directory
- Requests encodings [5, 1, 0] (Hextile, CopyRect, Raw)
- Tested against real OmniView: 10 frames in 10s, no errors

27 tests passing.

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

crates/ericrfb/src/codec/mod.rs

@@ -0,0 +1 @@
+pub mod hextile;