feat: SVG renderer for worksheet visual output

[Nebu1eto]

Summary

• Add render module in sheetkit-core that generates SVG from worksheet data (cell values, column widths, row heights, fonts, fills, borders, alignment, gridlines, headers)
• Expose Workbook.renderToSvg() method in both Rust facade and Node.js (napi-rs + TypeScript wrapper)
• Fix parse_style_color in napi conversions to accept raw ARGB/RGB hex strings (e.g. "FFFFFF00")
• Add EN and KO rendering guide documentation with feature list and known limitations

Test plan

• 29 Rust unit tests covering: valid SVG output, cell text, headers, gridlines, custom widths/heights, range rendering, empty sheets, bold/italic/underline/strikethrough, fills, font colors, alignment, borders, XML escaping, scale, shared strings
• 12 Node.js integration tests covering: valid SVG, cell text, gridlines toggle, headers toggle, sub-range, scale factor, error handling, bold style, fill colors
• cargo build --workspace passes
• cargo test --workspace passes (1300+ tests)
• cargo clippy --workspace passes (no warnings)
• cargo fmt --check passes
• pnpm build passes (napi + typecheck + tsdown)
• pnpm test passes (265 tests)

Generated with Claude Code

[Nebu1eto]

Review finding (blocking): RGB values that already include a leading # can become invalid ##RRGGBB in SVG output.

Relevant path: crates/sheetkit-core/src/render.rs (style_color_to_hex). Combined with accepted inputs in packages/sheetkit/src/conversions.rs (parse_style_color accepts #RRGGBB), this can generate invalid color attributes.

Please normalize colors before rendering (strip leading # or handle it explicitly).

[Nebu1eto]

Detailed review notes (blocking):

Renderer foundation is solid, but there is an SVG color correctness bug.

Blocking concern:

• parse_style_color accepts #RRGGBB input, while style_color_to_hex blindly prepends #.
• This can emit invalid values like ##RRGGBB in SVG attributes.

Relevant areas:

• crates/sheetkit-core/src/render.rs (style_color_to_hex)
• packages/sheetkit/src/conversions.rs (parse_style_color)

Additional issues worth fixing in this PR:

• If both underline and strikethrough are true, text-decoration is emitted twice; output should be merged into one valid value.
• Validate scale > 0 to avoid invalid/negative dimensions.

Please add direct tests for these rendering edge cases.