fix(bcf): export schema-valid BCF and read 3.0 colouring

[helenkwok]

Fixes #825.

• utils/xml.ts: XML.escape (the same helper as in fix(ids): decode XML entities on load and escape values on export #818, so the two merge cleanly).
• extensions.xml: XML declaration first, values escaped, placeholder schema location removed. documents.xml: Filename and Description escaped.
• bcf.version: empty <Version .../>.
• .bcfv: 3.0 <Color> wraps its components in <Components> (2.1 unchanged); AspectRatio written only for 3.0, after FieldOfView / ViewToWorldScale; AuthoringToolId as a child element; ids escaped.
• <Exceptions> uses exceptionComponents instead of repeating the selection.
• Viewpoint files are named <guid>.bcfv, and the markup references the same name.
• Import reads 3.0 <Color><Components> as well as 2.1 <Color><Component>.

Clipping planes in .bcfv, the toJSON snapshot encoding and 2.1 extensions are left for separate changes (listed in the issue).

Test

New src/openbim/BCFTopics/bcf-export.test.ts exports a topic with & and < in its type, label and document, a viewpoint titled Level 2/North with a selection, an exception and a colour, for both 2.1 and 3.0. It checks that every XML file is well-formed (fast-xml-parser's XMLValidator), the extensions file starts with the declaration, values are escaped, the viewpoint file is <guid>.bcfv at the topic root and matches the markup, the 3.0 and 2.1 colour and camera layouts, and that load() reads back the same colouring, selection and exceptions.

• With this change: 7 of 7 pass; packages/core vitest run 64 of 64; tsc --noEmit and eslint on the changed files report the same as main.
• Without it: all 7 fail.
• Outside the test, the exported 3.0 files (bcf.version, extensions.xml, documents.xml, markup.bcf, .bcfv) all validate with xmllint --schema against BCF-XML release_3_0/Schemas, and the 2.1 markup.bcf, .bcfv and bcf.version against release_2_1/Schemas.

The includeSelectionTag: true path (AuthoringToolId) needs a loaded model, so it is covered by reading the schema, not by the test.

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following:

• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Follow the Conventional Commits v1.0.0 standard for PR naming (e.g. feat(examples): add hello-world example).
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.