feat(nscodec): introduce ironrdp-nscodec crate + opt-in server integration (#1332)

Adds an opt-in implementation of the legacy RDP NSCodec encoder as a standalone crate, and wires it into `ironrdp-server` behind a feature flag so servers can serve NSCodec-only clients (notably macOS Microsoft Remote Desktop / Windows App) without default-build behavior changes.

Author: clintcan

Cargo.lock

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+[package]
+name = "ironrdp-nscodec"
+version = "0.1.0"
+readme = "README.md"
+description = "NSCodec ([MS-RDPNSC]) implementation for IronRDP"
+publish = false # TODO: publish
+edition.workspace = true
+license.workspace = true
+homepage.workspace = true
+repository.workspace = true
+authors.workspace = true
+keywords.workspace = true
+categories.workspace = true
+
+[lib]
+doctest = false
+# test = false # FIXME: turn off and keep tests in testsuite crates
+
+[features]
+# Encoder is opt-in. Default-disabled because consumers (notably
+# `ironrdp-server`) hide NSCodec behind their own feature gate too, so it
+# should never be pulled in incidentally.
+default = []
+encoder = ["dep:ironrdp-graphics"]
+
+[dependencies]
+ironrdp-graphics = { path = "../ironrdp-graphics", version = "0.8", optional = true } # public when `encoder` is on
+
+[lints]
+workspace = true

crates/ironrdp-nscodec/Cargo.toml

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+../../LICENSE-APACHE

crates/ironrdp-nscodec/LICENSE-APACHE

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+../../LICENSE-MIT

crates/ironrdp-nscodec/LICENSE-MIT

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+# ironrdp-nscodec
+
+NSCodec ([MS-RDPNSC]) implementation for IronRDP.
+
+NSCodec is a legacy bitmap codec used in the RDP "Surface Bits" command path. It
+predates RemoteFX but remains the only legacy codec advertised by the macOS
+Microsoft Remote Desktop / Windows App client's bitmap codec list, so servers
+wanting non-raw bitmap delivery to that client need it.
+
+## Feature Flags
+
+- **`encoder`** -- Opt-in; pulls in the server-side encoder
+  (`ironrdp_nscodec::encoder::encode`) and `ironrdp-graphics` for the
+  `PixelFormat` input enum.
+
+With no features (`default-features = false`), the crate compiles to an empty
+shell — enable `encoder` to get the actual code.
+
+## Status
+
+Encoder side only. Implements the codec defined in MS-RDPNSC §3.1.5:
+
+1. RGB → YCoCg color-space conversion (lossy on chroma when CLL > 0).
+2. Per-plane RLE compression (custom MS-RDPNSC byte-level RLE).
+3. 20-byte frame header + concatenated Y, Co, Cg, A planes.
+
+Chroma subsampling (`ChromaSubsamplingLevel = 1`, 4:2:0) is not yet
+implemented; the encoder always emits `ChromaSubsamplingLevel = 0`.
+
+[MS-RDPNSC]: https://learn.microsoft.com/en-us/openspecs/windows_protocols/ms-rdpnsc/68df0993-2c44-4d57-8aef-cdab1c1c43a8