Support Marpit  syntax for slide background images

[Copilot]

![bg](url) was treated as a normal inline image instead of a slide background. This adds parser recognition of the bg alt-text marker and wires it into the existing background rendering path.

How it works

# Slide Title

![bg](photo.jpg)

![bg](url) is now promoted to SlideStyle.BackgroundImage during parsing and removed from the slide element list — the image renders identically to a <!-- backgroundImage: url --> directive.

Changes

Parser (MarpMarkdownParser.cs)

• After ParseElements(), collects any ImageElement with alt text exactly "bg" (case-insensitive)
• Applies the first one to SlideStyle.BackgroundImage unless a directive has already set it
• Removes all bg-marker elements from the slide element list

Precedence

• Directive (<!-- backgroundImage: ... --> / front-matter) > ![bg](url) syntax > theme CSS
• When multiple ![bg](url) appear on one slide, only the first is used; split-background layouts are not yet supported

Tests

• Renamed existing tests that captured old "inline picture" behavior to reflect the new semantics
• Added 6 parser tests: background image set, element not added, directive precedence, other elements preserved, case insensitivity, first-of-multiple used
• Added 1 renderer test: directive wins over ![bg](...)

Docs (doc/marp-markdown.md)

• Feature table updated to Supported subset
• New Background Image Precedence section documents the three-tier priority
• Not Yet Supported section updated to list only the unimplemented modifiers (bg cover, bg contain, bg left, bg right, percentage sizing, split layouts)

[Copilot]

Pull request overview

This PR adds Marpit-style background image support via Markdown image alt text (![bg](url)), promoting those images to SlideStyle.BackgroundImage so they render as full-slide backgrounds rather than inline image elements.

Changes:

• Update the core Markdown parser to detect ![bg](...) images (case-insensitive bg) and set the slide background image accordingly, removing those images from slide elements.
• Update/extend parser and renderer tests to assert the new ![bg] background behavior and precedence with backgroundImage directives.
• Update documentation to describe the supported subset and precedence rules for background images.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
tests/MarpToPptx.Tests/PptxRendererTests.cs	Adjusts/extends renderer assertions so ![bg] results in a background picture (empty Description) and validates directive-vs-syntax behavior.
tests/MarpToPptx.Tests/ParserTests.cs	Updates the prior “bg keyword as alt text” expectation to “sets slide background”, and adds a suite of ![bg] parser behavior tests (precedence, element removal, case-insensitivity, first-only).
src/MarpToPptx.Core/Parsing/MarpMarkdownParser.cs	Implements promotion of  images to SlideStyle.BackgroundImage and skips adding those images as inline elements.
doc/marp-markdown.md	Documents  support, limitations (modifiers not parsed, first-only), and background image precedence.

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[jongalloway]

@copilot apply changes based on the comments in this thread

[Copilot]

@copilot apply changes based on the comments in this thread

Both issues from the review are fixed in 05985e7:

1. Type-safe filter — replaced bgImages.Contains(element) with element is ImageElement img && IsBgAltText(img.AltText) in the element-filtering loop.
2. Empty-directive precedence — changed string.IsNullOrWhiteSpace(effectiveStyle.BackgroundImage) to effectiveStyle.BackgroundImage is null, so an explicit empty-value directive (<!-- backgroundImage: -->) correctly blocks ![bg](...) promotion on the same slide. The comment now accurately says "always takes precedence". Added a new parser test (Parser_BgImageSyntax_EmptyDirectiveClearsBackground_BgSyntaxDoesNotPromote) to cover this case.