Resolve percent <svg> dimensions against --size and improve error

[rursache]

Summary

Issue #42 covers SVG canvas units. PR #45 added support for absolute units on the root <svg> width/height. Two cases still failed at parse time with a generic missingAttribute error:

1. Percent values, eg width=\"100%\", when no viewBox is present
2. Missing width/height entirely when no viewBox is present

Per the SVG spec these need a viewport to resolve against. SwiftDraw had no way for callers to supply one, so this PR adds an opt-in viewport and threads --size through the CLI so the existing flag becomes the user-visible answer.

What changes

• XMLParser gains an XMLParser.Viewport struct and a defaultViewport property
• DOM.SVG.parse(fileURL:options:defaultViewport:) and parse(data:options:defaultViewport:) accept the viewport
• The CLI passes config.size as the viewport when loading SVGs for png/jpeg/pdf output, so --size 800x600 responsive.svg resolves percent dimensions and missing dimensions against the requested canvas
• Falls back to viewBox dimensions when width/height are missing but a viewBox is declared (already common browser behavior)
• New XMLParser.Error.unresolvableDimension(reason:) replaces missingAttribute for these cases. The reason string explains exactly what is missing and how to fix it, eg:

  <svg> width=\"100%\" cannot be resolved without a viewBox or an explicit viewport (--size on the command line)

• Public SVG initializers and existing API surface are unchanged. Callers that do not opt in to a viewport now get the clearer error in place of the previous one
• README documents the new resolution behavior under the command line tool section

Examples

Before:

$ swiftdraw responsive.svg --format png --size 800x600
# fails: missingAttribute(\"width\")

After:

$ swiftdraw responsive.svg --format png --size 800x600
# resolves width=\"100%\" against 800x600 and renders

SVGs that omit dimensions but provide a viewBox now also work without --size:

<svg viewBox=\"0 0 150 75\">...</svg>

Test plan

• New unit tests in DOM/Tests/Parser.SVGTests.swift covering: percent + viewport, percent without viewport (throws), missing dims + viewport, missing dims + viewBox, missing all (throws unresolvableDimension), mixed percent/explicit
• Existing parseSVGMissingHeightInvalidNode / parseSVGMissingWidthInvalidNode still throw (unchanged behavior when no viewport is provided)
• Full suite: 207 tests pass (swift test)
• swift build clean

Relates to #42

[codecov]

Codecov Report

❌ Patch coverage is 84.21053% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.76%. Comparing base (8f03233) to head (1e8f3a6).

Files with missing lines	Patch %	Lines
SwiftDraw/Sources/SVG.swift	0.00%	12 Missing ⚠️
DOM/Sources/Parser.XML.SVG.swift	92.30%	3 Missing ⚠️
DOM/Tests/Parser.SVGTests.swift	96.15%	2 Missing ⚠️
...Draw/Sources/CommandLine/CommandLine+Process.swift	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #126      +/-   ##
==========================================
- Coverage   89.80%   89.76%   -0.05%     
==========================================
  Files         159      159              
  Lines       15868    15961      +93     
==========================================
+ Hits        14251    14327      +76     
- Misses       1617     1634      +17     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.