[Enhancement] Multi-layer page border support for `.pageformat`

[CarmJos]

Before submitting

• I am using the latest version of Quarkdown.
• I have checked the wiki to confirm this feature doesn't already exist.
• I have searched existing issues for duplicates.

Description

Currently, .pageformat's bordertop, borderright, borderbottom, and borderleft parameters accept a single Size value (e.g. 4px, 1cm), which only allows a simple solid border with uniform thickness. This is insufficient for users who want richer page border styles commonly found in word processors like Microsoft Word.

This proposal introduces multi-layer border support, enabling users to define compound borders composed of multiple concentric layers from outer to inner, each with its own width and style. It also adds a bordermargin parameter to control the offset of the border from the content area edge, and expands the set of available border styles with fine-grained control options.

Proposed syntax or behavior

1. Multi-layer border syntax

Each border side parameter (bordertop, borderright, borderbottom, borderleft) should accept a comma-separated list of border layers. Each layer follows the format:

<width> [style] [style-specific-options...]

Layers are rendered from outer to inner (first layer = outermost). Example:

.pageformat bordertop:{4px, 1px none, 2px dotted}

This creates, from outside to inside:

1. 4px solid border
2. 1px gap (no border)
3. 2px dotted border

A single value (the current syntax) should remain valid as shorthand for a single-layer border:

.pageformat bordertop:{4px}

2. Border styles

The following styles should be supported. When omitted, the default is solid.

Style	CSS equivalent	Description
solid	border-style: solid	Solid line (default)
dotted	border-style: dotted	Dotted line
dashed	border-style: dashed	Dashed line
waved	custom	Wavy line (no CSS equivalent; requires custom rendering)

3. Style-specific options

Each style supports optional parameters for fine-grained control. These are specified as key-value pairs after the style name.

dotted -- controls dot size and spacing:

2px dotted [dotsize] [spacing]

dashed -- controls dash length and spacing:

2px dashed [length] [spacing]

waved -- controls wave amplitude and period:

2px waved [amplitude] [period]

4. New bordermargin parameter

A new bordermargin parameter should be added, accepting 4 Size values (top, right, bottom, left), to control the distance between the border and the page content edge:

.pageformat bordermargin:{10px 15px 10px 15px}

This maps to padding/margin between the content area boundary and the border rendering position.

5. Backward compatibility

• A single Size value (e.g. 4px) should continue to work as a single solid layer.
• bordercolor should apply as the default color for all layers that do not specify their own color.
• If bordercolor is not set, the default foreground text color is used (existing behavior).

6. Example

.pageformat \
    bordertop:{4px solid, 1px none, 2px dotted 3px} \
    borderbottom:{3px waved 4px 8px} \
    borderleft:{2px dashed 10px 5px} \
    bordercolor:{Red} \
    bordermargin:{10px 15px 10px 15px} \

Additional context

• The primary inspiration is Microsoft Word's Page Border feature (Design > Page Borders), which supports box, shadow, 3-D, and custom borders with styles including solid, dashed, dotted, double, wavy, and more, with options for color, width, and art patterns.
• The waved style has no direct CSS border-style equivalent and will require custom rendering, likely via SVG border-image or a background pattern. This may be the most complex part of the implementation.
• The dotted and dashed style-specific options (dotsize, length, spacing) can leverage CSS border-image with a repeating pattern or background-image with repeating-linear-gradient for precise control, since CSS border-style: dotted/dashed offers no native spacing control.
• The existing Container.BorderStyle enum (NORMAL, DASHED, DOTTED, DOUBLE) in quarkdown-core is a separate system for container borders and does not map directly to this feature, but the naming conventions should be kept consistent.