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/session.rs

@@ -1,6 +1,7 @@
 use std::io::{BufReader, BufWriter};
 use std::net::TcpStream;
 
+use crate::codec::hextile;
 use crate::framebuffer::Framebuffer;
 use crate::handshake::{self, Config, ServerInit};
 use crate::msg::{self, ServerMsg};
@@ -196,6 +197,17 @@ impl ActiveSession {
                     self.framebuffer
                         .copy_rect(src_x, src_y, hdr.x, hdr.y, hdr.w, hdr.h);
                 }
+                5 => {
+                    // Hextile
+                    hextile::decode_hextile(
+                        &mut self.reader,
+                        &mut self.framebuffer,
+                        hdr.x,
+                        hdr.y,
+                        hdr.w,
+                        hdr.h,
+                    )?;
+                }
                 other => {
                     return Err(SessionError::UnsupportedEncoding(other));
                 }