docs(server/scene): mark ExternalPosition2D as an unimplemented stub

[joschaschmiedt]

Marks the ExternalPosition2D animation as an unimplemented stub at every site that currently claims it reads shared memory. No behaviour change.

Why

The server accepts an ExternalPosition2D animation, returns a valid handle, reports Running from QueryAnimation, and round-trips it through config save/load. The stimulus never moves. The entire implementation is animation_advance.rs:

// External position is driven by an external process; never self-terminates.
Animation::ExternalPosition2D { .. } => false,

No shm_open, no mmap, no move_to. The comment describes an intent, not a behaviour.

Everything around the read exists, which is what makes it hard to notice — the enum variant, the proto message, the command.rs create/query wiring and the Python client method all ship today. Three of them promise a per-frame shm read in their doc comments. A client author reading animations.proto or the Python docstring has no way to discover the command does nothing; they get a success response and a motionless stimulus, with nothing in the log.

So this marks all four sites rather than only the Rust arm:

• server/src/scene/animation/animation_advance.rs — the stub match arm
• server/src/scene/animation/animation_kind.rs — "Read 2-D position from a POSIX shm float array each frame."
• proto/vstimd/v1/animations.proto — the same claim on the wire type
• client/python/vstimd/animations/animations_client.py — the Python docstring, as a Sphinx .. warning::

Scope

Documentation only. Deciding whether to reject at create time with UNIMPLEMENTED or implement the read is #84; that issue argues for rejecting if #79 is close and implementing if it is not, but rules out leaving it accepting-and-ignoring.

Not folded into #79 because that issue implements this arm as one part of a feature depending on the vinput crate (#77) and the device registry (#78), inside epic #81. The defect — a shipped command that silently does nothing — stands independently of whether that epic lands.

Verification

• cargo clippy clean (the one warning, module_inception in scene/stimulus/mod.rs, is pre-existing)
• make typecheck passes
• No generated *_pb2.* files staged

Refs #84, #79, #83

🤖 Generated with Claude Code

https://claude.ai/code/session_01TovGgCFRVaGjUhKHLVKgUF