docs: correct the sulcus install instructions, which destroyed existing sulci

[jackgallant]

Follow-up to #656. Docs-only, no code touched.

The bug

The roidraw page tells users to "Merge the fragment into the subject's overlays.svg". Taken
literally — which is how it reads — that means appending the exported <g id="sulci"> layer to the
subject's overlay file. But SVGOverlay.__init__ keys its layers by inkscape:label:

for layer in self.svg.getroot().findall("{%s}g"%svgns):
    layer = Overlay(self, layer)
    self.layers[layer.name] = layer      # <- a second "sulci" layer replaces the first

So the appended layer silently replaces the subject's own, and every sulcus already in that file
becomes invisible to db.get_overlay(), quickflat, and the WebGL viewer. Nothing errors; the
sulci just stop existing.

The correct instruction is to copy the individual <g inkscape:label="…"> shape groups into the
subject's existing #sulci_shapes group. This PR says so, in a .. warning::, with a worked
example showing which element to copy.

Two more corrections, both from pycortex-roidraw v0.4.1

The page was written against v0.4.0, whose exporter has since been fixed. It now describes output
that no longer exists:

• The export is no longer a bare fragment. It is a standalone SVG document declaring the svg
  and inkscape namespaces. The old fragment used the inkscape: prefix without declaring it, so
  ET.parse failed with unbound prefix and no XML parser — Python, lxml, a browser, Inkscape —
  could open the file at all.
• The exported sulci_labels layer is empty, and the page now says why. pycortex derives each
  sulcus's label position from its path geometry at load (Shape.get_labelpos, one label per path,
  so a two-hemisphere sulcus is labelled twice for free) and writes data-ptidx itself in
  SVGOverlay.set_coords. data-ptidx is pycortex's output, not its input — a real overlays.svg
  contains zero <text> elements. v0.4.0 wrote <text data-ptidx="…"> labels with no x/y, and
  Labels.__init__ does an unguarded float(text.get('x')) over every <text> in a labels layer,
  so db.get_overlay(subject) raised TypeError before rendering anything.

Verification

Every API call in the new example was checked against this repo's source rather than recalled:

in the docs	in the source
cortex.db.get_overlay('S1')	cortex/database.py:349
svg.sulci	SVGOverlay.__getattr__, cortex/svgoverlay.py:99
svg.sulci['CS']	Overlay.__getitem__, cortex/svgoverlay.py:338
make_figure(…, with_sulci=True)	cortex/quickflat/view.py:37

The reStructuredText was parsed with docutils (Sphinx-only roles stripped) and is clean; I confirmed
that check can fail by feeding it a short section underline.

Sulcus drawing itself is unaffected — this is the documentation catching up with
pycortex-roidraw v0.4.1.

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request updates the documentation in docs/roidraw.rst to clarify that drawn sulci are exported as a standalone SVG rather than an SVG fragment. It adds a new section, "Installing drawn sulci into a subject," which provides step-by-step instructions, XML and Python examples, and a warning about correctly merging the exported groups into a subject's existing overlays.svg file. No review comments were provided, so there is no feedback to address.

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