Apply context-sensitive resolution (CSR) consistently in MarkdownRendering.kt

[morisil]

Summary

markanywhere-render/src/commonMain/kotlin/MarkdownRendering.kt applies Kotlin context-sensitive resolution (CSR) inconsistently: some when/lambda sites over a BlockFrame subject drop the BlockFrame. prefix (e.g. is Pre, is Code, is Inline, is Frontmatter), while structurally identical sites still use the fully-qualified BlockFrame.X. Per CLAUDE.md ("Inside a when whose subject has a known sealed type … drop the type prefix … CSR also applies to is/as against that type"), these should be consistent. This is a style/readability cleanup, surfaced during review of #57 (which added several of the unqualified forms next to pre-existing qualified ones).

Already unqualified (correct, the target style)

• blockMarkerEscapingAllowed() — it is Pre || it is Code || it is Frontmatter
• the blank-text dispatch — it is Pre || it is Code
• enclosingInlineDelimiterMask / inInlineCode — is Inline, is TaggedInline, is Code
• the inline-code early-return — inInlineCode() (was blockStack.lastOrNull() is Code)

Still fully-qualified (to normalize)

In MarkdownRendering.kt (line numbers approximate, will drift):

• buildPrefix when (subject BlockFrame): is BlockFrame.Blockquote, is BlockFrame.List, (next as? BlockFrame.ListItem)
• spillActiveLinks: frame !is BlockFrame.Link
• the block-spill guard: blockStack.any { it is BlockFrame.Link && !it.rawMode }
• tagged Unmark path: frame is BlockFrame.SelfClosed, frame is BlockFrame.TaggedInline
• list handling: parent is BlockFrame.ListItem, blockStack.lastOrNull() is BlockFrame.ListItem, (blockStack.last() as BlockFrame.ListItem), blockStack.lastOrNull() as? BlockFrame.List, blockStack.lastOrNull() is BlockFrame.Pre
• the Unmark when (frame.name) casts: frame as BlockFrame.Frontmatter, (frame as BlockFrame.Paragraph).suppressClose, frame as BlockFrame.Inline, frame as BlockFrame.Link
• "ul"/"ol" close: blockStack.lastOrNull() is BlockFrame.ListItem

Notes / caveats

• CSR applies to is/as against the sealed type and inside a when whose subject is the sealed type. For a nullable receiver (blockStack.lastOrNull() as? BlockFrame.List) and bare as casts, confirm CSR resolves before mass-replacing — compile each target. CLAUDE.md notes CSR does not apply in some positions (supertype lists, generic constraints, type-annotation positions), so this is a per-site check, not a blind find/replace of BlockFrame..
• Construction sites (addLast(BlockFrame.Inline(...)), BlockFrame.SelfClosed as a value, etc.) are not CSR positions and must stay qualified.

Scope

Pure refactor, no behavior change; verify by running :markanywhere-render:jvmTest. Kept out of #57 to avoid ballooning that PR's diff with churn in untouched code.