feat: add bitmap surface protocol and renderer

[gold-silver-copper]

Summary

This replaces #133 on current main and completes the Rio VT ownership migration begun in #140.

• add the Ratty Bitmap Surface Protocol v1 parser, bounded transfer/lifecycle state, renderer, examples, configuration, and protocol documentation
• make rio-vt the sole owner of terminal parsing, external placement mutation, Kitty wire parsing, image/placement lifetime, and replies
• consume Rio's terminal-owned placement geometry for Ratty bitmap/RGP objects across partial/full scrolling, history eviction, resize/reflow, and screen swaps
• render Rio's native direct and Unicode-placeholder Kitty placements with stable, bounded Bevy image assets
• enforce encoded/decoded/resident/pending object limits, incomplete-transfer expiry, checked dimensions, explicit direct-only Kitty media policy, and bounded synchronous RGP mesh decoding

Rio dependency

Ratty pins the coordinated draft raphamorim/rio#1859 at immutable revision 535495772f8548e2cbf9a840938c8eca21714888.

That upstream change supplies the supported external-placement API and corrects native Kitty placement/deletion/retransmission/resource semantics. Ratty no longer reconstructs VT scrolls or maintains a second Kitty protocol state machine.

Verification

• cargo +1.96.1 fmt --all --check
• cargo +1.96.1 check --all-targets --locked
• cargo +1.96.1 clippy --all-targets --locked -- -D warnings
• cargo +1.96.1 test --all-targets --locked — 231 library, 7 bitmap-frame example, and 4 pan/zoom example tests passed
• RUSTDOCFLAGS='-D warnings' cargo +1.96.1 doc --locked --no-deps
• widget fmt --all --check, check, and clippy --all-targets -- -D warnings
• git diff --check

Automated stream fixtures cover Kitty 0.46.1-compatible unpadded RawStd, zlib, chunking, capability queries, and Unicode placeholders. Manual GPU/visual validation with Kitty 0.46.1 remains outstanding.

Review notes

The complete 22-file diff was reviewed against merge base cb0b625ae2ed92ea23a3651b17c110a1e8130eb5, including the coordinated Rio diff. Resource-exhaustion, Kitty deletion/retransmission/capacity/ID-lifetime, file/shared-memory bounds, alternate-screen/ED2 lifetime, RGP parser/decoding ownership, and stale-transfer findings were fixed and covered by regressions.

Two renderer constraints are documented: partially clipped arbitrary 3D RGP meshes keep their full model transform and do not yet receive an exact per-placement fragment scissor; ordinary negative Kitty z-order cannot be placed between cell backgrounds and glyphs while those layers remain combined in one terminal texture. Extreme-negative/default-background behavior and Kitty-to-Kitty z ordering are preserved.