CI: compile/test a feature matrix including `arbitrary` (std/no_std)

[CBenoit]

Prevent regressions by ensuring CI compiles key crates under important feature combinations.

Approach

Two acceptable options:

1. Use cargo-hack to run a selected matrix, OR
2. Add explicit cargo check commands for curated feature sets

Tasks

• Decide matrix strategy (cargo-hack vs curated commands)
• Add CI job(s) to cover at least:
  • --features arbitrary for PDU/protocol crates
  • std enabled and std disabled (no_std/alloc) where supported
• Ensure runtime stays reasonable (can be a separate, parallel job)

Example of the kind of coverage we want (exact command can be refined):

• cargo hack check --workspace --each-feature --no-dev-deps
  or a curated set like:
• cargo check -p ironrdp-core --no-default-features --features alloc
• cargo check -p ironrdp-core --features std
• cargo check -p ironrdp-pdu --features std
• cargo check -p ironrdp-pdu --features arbitrary

Acceptance criteria

• CI fails if arbitrary breaks
• CI validates at least one no_std/alloc build path if supported
• Matrix is documented (so future contributors know what to extend)