Make font-holding config structs Sendable via SendableFont wrapper

[Jun Yan (junyan72)]

Summary

Make the font-holding configuration structs safely Sendable under strict concurrency by centralizing the unchecked assertion in a single internal wrapper, instead of applying @unchecked Sendable to each struct.

• Add internal SendableFont value type (Utilities/SendableFont.swift) that wraps MDFont (UIFont/NSFont) and carries the sole @unchecked Sendable assertion, documented in one auditable place.
• TextFonts, MarkdownRenderConfig.MarkdownInlineTextStyle, and MarkdownRenderConfig.CitationConfig now store SendableFont privately and expose their existing public MDFont API via computed accessors. The structs stay declared Hashable, Sendable (unchanged public surface, no @unchecked).
• No consumer source break: public initializers and font property types are identical; MDFont remains the native alias everywhere in the rendering path (NSAttributedString boundaries, font metrics) and in tests.
• Avoids leaking a retroactive Sendable conformance onto the platform font types.

Closes #124

Validation

• xcodebuild build -scheme SwiftStreamingMarkdown -destination "platform=iOS Simulator,name=iPhone 17" -skipMacroValidation SWIFT_STRICT_CONCURRENCY=complete — BUILD SUCCEEDED, zero errors and zero font/SendableFont Sendable warnings (remaining strict-concurrency warnings are unrelated: LaTeX regexes, view controllers, TableView mocks).
• swiftlint --strict on the three changed/added files — 0 violations.
• Existing test suite unaffected: tests use MDFont factory methods, which still resolve to the native font type.

OSS readiness

• No secrets, internal URLs, private identifiers, or product-only service names were added.
• Public docs, fixtures, or notices were updated if behavior or dependencies changed. (public API surface unchanged; no docs update needed)
• Third-party dependency changes (adds, removes, version bumps) are intentional and reviewed. (no dependency changes)
• Streaming/incomplete markdown behavior remains covered by fixtures or tests. (no rendering-path changes)

[Copilot]

Pull request overview

This PR centralizes the single @unchecked Sendable assertion needed for platform fonts (UIFont/NSFont) into an internal SendableFont wrapper so public configuration types can remain plainly Sendable under strict concurrency without scattering @unchecked Sendable across multiple structs.

Changes:

• Added internal SendableFont wrapper that holds MDFont and carries the only @unchecked Sendable conformance.
• Updated TextFonts to store fonts privately as SendableFont/SendableFont? while preserving the existing public MDFont API via computed accessors.
• Updated MarkdownRenderConfig.MarkdownInlineTextStyle and MarkdownRenderConfig.CitationConfig to use private SendableFont storage and computed MDFont accessors, keeping public initializer signatures and types unchanged.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
Sources/MarkdownText/Utilities/SendableFont.swift	Introduces a single internal wrapper type that centralizes the unchecked Sendable assertion for platform fonts.
Sources/MarkdownText/Style/TextFonts.swift	Moves stored font variants to private SendableFont backing storage while exposing the same public MDFont-typed API.
Sources/MarkdownText/Models/MarkdownRenderConfig.swift	Updates inline text style and citation config to store fonts via SendableFont backing fields and keep public MDFont accessors/initializers.