Fix pipeline module alignment and run_likelihood hang

[Copilot]

Pipeline module boxes were left-aligned, causing connectors to be diagonal when modules had different widths. Running a likelihood evaluation could hang indefinitely due to cosmosis child processes keeping a pipe write-end alive.

Pipeline module centre alignment (pipeline.js)

Added _modLeft(module) helper that offsets each module's left edge so all boxes share a single centre x:

_modLeft(module) {
  return this._modX() + (this._maxModWidth() - this._moduleBoxWidth(module.name)) / 2;
}

• _appendModule, _appendPorts, _appendSourceOutputBoxes, _computeSourceOutputPositions, _appendPortEdges — all updated to use _modLeft(module) for the module's left edge
• _appendConnections — simplified to a single cx = _modX() + maxModWidth/2 for both endpoints → perfectly vertical arrows

Fix run_likelihood hang (worker.py)

_FdCapture uses a pipe + drain thread. CosmoSIS spawns child processes (OpenMP, external programs) that inherit the pipe write end; when the parent restores stdout/stderr the children keep the write end alive, so the drain thread never sees EOF and the worker blocks.

• run_likelihood: replaced _FdCapture with new _FdDevNull — redirects fd 1/2 to /dev/null instead of a pipe. Writes are instant and non-blocking; child fd inheritance is harmless. The captured text was unused on the success path anyway.
• _FdCapture (still used in prepare_pipeline): saves self._r, reduces initial join timeout to 5 s, then force-closes the pipe read end to unblock the drain thread, with a final 3 s join — worst-case delay is now ~8 s instead of indefinite.

Original prompt

I'd like to create in this new new repo and package that provides a GUI to my python package CosmoSIS, which is in the repository cosmosis-developers/cosmosis.

I'd like the interface to be in a web browser, so all the UI logic to be in javascript. CosmoSIS is in python, and I'd like to use flask with websockets to do the communication between the two.

To start with let's do that basic infrastructure, and try out UI idea.s

I'd like there to be a tab bar at the top of the main UI page with different contents that we will work on one-by-one. To start with I'd like to work on the first tab, called "Pipeline". As a placeholder, please add other tabs called "Results", "Sampling" and "Campaign", but they can be empty for now.

So first we can design the first tab, "Pipeline". This main area of this tab should be a canvas which will display a representation of a workflow which is a sequence of "modules" that are to be run. Each module has some inputs and some outputs which should be displayed like flow chart items flowing into and out of the module when the user clicks on the module.

The python side will be supplying the list of available modules and their inputs and outputs, but for now should just supply some placeholders.

For the inputs that are displayed when the user clicks, the corresponding earlier module that created them as an output should also show as an edge on the chart.

You can use any open source library you want for the chart, or just roll your own, whichever suits better.

The canvas should initially have one single module in, called "Sampler", which will always be present and can't be removed.

In a sidebar on the left we will display a library of components that the user can drag onto the canvas. These components form a linear sequence, so when a user drags them in they should slot into the pipeline between any existing elements. The linear sequence should be evenly spaced.

There should also be a sidebar on the right that only opens when the user clicks on a module, either in the left sidebar or in the main canvas. We'll work on that later.

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