Add configurable D2 SVG output with bundling support

[mnowotnik]

Changes:

• d2/d2.go: add SVG bundling via imgbundler, scale-aware dimensions, and adapter logger
• renderer/fencedcodeblock.go: pass file path, bundle flag, and scale through SVG code path
• markdown/markdown.go: wire new fenced-code renderer signature
• util/flags.go, util/cli.go, types/types.go: introduce d2-output/d2-bundle config and propagate to MarkConfig
• d2/d2_test.go: cover SVG path and checksum expectations
• README.md: document new d2-output and d2-bundle options
• go.mod/go.sum: pull in new indirect deps
• main.go: remove stray blank line

Why:

• Allow uploading original D2 SVGs with optional bundled assets and respect scaling to avoid manual resizing.