feat(pdu,graphics,egfx): add ClearCodec bitmap compression codec

[Greg Lamberson (glamberson)]

Part of the multi-codec EGFX implementation described in #1158 (Section 3).

Summary

Add ClearCodec (MS-RDPEGFX 2.2.4.1), the mandatory lossless bitmap codec
for all EGFX versions (V8-V10.7). ClearCodec uses three-layer compositing
(residual BGR RLE, bands with V-bar caching, subcodec regions) to encode
text, UI elements, and icons with pixel-perfect fidelity.

This is the first ClearCodec encoder in any open-source RDP implementation.
FreeRDP has client-side decode only.

What it adds

ironrdp-pdu (wire format):

• codecs/clearcodec/: ClearCodecBitmapStream, CompositePayload, residual
  RLE decode/encode, bands with V-bar cache references (full hit, short
  hit, short miss), RLEX subcodec, raw/NSCodec dispatch
• 24 inline unit tests for wire format parsing

ironrdp-graphics (codec logic):

• clearcodec/: ClearCodecDecoder with persistent V-bar and glyph caches,
  ClearCodecEncoder with residual-only encoding and glyph deduplication
• VBarCache: 32,768 full + 16,384 short V-bar ring buffers per spec
• GlyphCache: 4,000 entries, area <= 1024 pixels per spec
• 18 inline unit tests for cache operations

ironrdp-egfx (server integration):

• GraphicsPipelineServer::send_clearcodec_frame()

ironrdp-testsuite-core:

• 35 integration tests: codec round-trip with pixel-perfect verification,
  adversarial input handling (max run_length, OOM dimensions, malformed
  streams, uncached glyph hits), bands layer compositing through the full
  decode pipeline, cache state management across multi-frame sessions,
  compression quality assertions

Security hardening

Three rounds of code review against FreeRDP ClearCodec CVEs:

• Cap residual run_length to output buffer (GHSA-32q9-m5qr-9j2v)
• Cap allocation to 8192x8192 pixels (CVE-2026-26955 class)
• Fix shortVBarYOff: absolute end-row offset, not pixel count delta
  (MS-RDPEGFX 2.2.4.1.1.2.1.1.3)
• Validate glyph index range 0-3999 (CVE-2026-23531 class)
• Validate shortVBarYOff >= shortVBarYOn

NSCodec

The NSCodec subcodec (Layer 3 option) is not implemented in this PR.
The encoder avoids generating NSCodec tiles, and the decoder stub leaves
NSCodec regions at their residual-layer values. This can be added later
without API changes.

Test plan

• cargo xtask check fmt/lints/tests/typos/locks (all pass)
• 77 total tests (35 testsuite + 42 inline)

[Copilot]

Pull request overview

Adds ClearCodec (MS-RDPEGFX 2.2.4.1) support across the wire-format layer (ironrdp-pdu), codec logic (ironrdp-graphics), and EGFX server API (ironrdp-egfx), with extensive new tests in ironrdp-testsuite-core.

Changes:

• Introduces ironrdp-pdu::codecs::clearcodec with residual/bands/subcodec parsing and helpers.
• Adds ironrdp-graphics::clearcodec encoder/decoder plus persistent V-bar and glyph caching.
• Extends GraphicsPipelineServer with send_clearcodec_frame() and adds ClearCodec integration tests.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
crates/ironrdp-testsuite-core/tests/graphics/mod.rs	Registers the new ClearCodec integration test module.
crates/ironrdp-testsuite-core/tests/graphics/clearcodec.rs	Adds ClearCodec end-to-end and adversarial integration tests.
crates/ironrdp-pdu/src/codecs/mod.rs	Exposes the new ClearCodec codec module.
crates/ironrdp-pdu/src/codecs/clearcodec/mod.rs	Defines ClearCodec bitmap stream + composite payload parsing and exports layer decoders.
crates/ironrdp-pdu/src/codecs/clearcodec/residual.rs	Implements residual (BGR RLE) encode/decode.
crates/ironrdp-pdu/src/codecs/clearcodec/bands.rs	Implements bands layer decoding (V-bar references + inline data).
crates/ironrdp-pdu/src/codecs/clearcodec/subcodec.rs	Implements subcodec layer parsing (raw/NSCodec/RLEX region headers).
crates/ironrdp-pdu/src/codecs/clearcodec/rlex.rs	Implements RLEX subcodec parsing.
crates/ironrdp-graphics/src/lib.rs	Exposes the new clearcodec module.
crates/ironrdp-graphics/src/clearcodec/mod.rs	Implements ClearCodec decoder/encoder and compositing across layers.
crates/ironrdp-graphics/src/clearcodec/glyph_cache.rs	Adds glyph cache backing storage used by encoder/decoder.
crates/ironrdp-graphics/src/clearcodec/vbar_cache.rs	Adds V-bar/short V-bar cache backing storage used by decoder.
crates/ironrdp-egfx/src/server.rs	Adds send_clearcodec_frame() EGFX server API entry point.

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