Fixes #4955. Replace regex-based Markdown lowering with Markdig AST walker

[Copilot]

The MarkdownPipeline property was a dead API — Markdig's AST was parsed and immediately discarded, with all rendering driven by regex on raw source text. This meant custom pipeline settings had no effect, and several Markdown constructs were broken or missing entirely.

Core Change

LowerFromSourceText() (4 static Regex patterns + line-by-line processing) replaced by LowerFromAst(MarkdownDocument) that walks Markdig's parsed AST:

• WalkBlock() — dispatches all block types: HeadingBlock, ParagraphBlock, FencedCodeBlock/CodeBlock, ThematicBreakBlock, QuoteBlock, ListBlock, Table, HtmlBlock
• WalkInlines() — dispatches all inline types: LiteralInline, EmphasisInline, CodeInline, LinkInline, AutolinkInline, LineBreakInline, HtmlEntityInline, with ContainerInline fallback for unknown types (e.g. PipeTableDelimiterInline)
• Blank-line preservation via source line-number gap analysis between consecutive top-level blocks
• Task list detection via TaskList inline (Markdig extension)
• Table Option A: extract cell text from AST → feed existing MarkdownTable re-parse path

New Strikethrough Role

Added MarkdownStyleRole.Strikethrough (maps to TextStyle.Strikethrough) for EmphasisInline with DelimiterChar == '~' && DelimiterCount >= 2.

Newly Unlocked Behaviors

Feature	Before	After
MarkdownPipeline property	Dead API	Controls rendering
~~strikethrough~~	Literal tildes	Strikethrough style
Setext headings (Title\n=====)	Broken	Anchor + heading
Indented code blocks (4 spaces)	Broken	MarkdownCodeBlock SubView
Autolinks (<https://...>)	Broken	Link role
HTML entities (&amp;)	Literal	Transcoded character
Escaped chars (\*)	Broken	Correct pass-through

Test Updates

• 56 new tests in AstLoweringTests.cs (baseline coverage, formerly-skipped known-limitation tests, per-phase unit tests)
• Invalid_Table_Lines_Rendered_As_Text: relaxed >= 3 line assertion — Markdig correctly parses pipe-lines without a separator row as a single paragraph, not 3 separate paragraphs

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
• My code follows the style guidelines of Terminal.Gui - if you use Visual Studio, hit CTRL-K-D to automatically reformat your files before committing.
• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working

Original prompt

use a branch name of "md-pipeline-refactor"

[Chronological Review: 1. User asked to study ./plans directory and summarize attack approach 2. Agent read the ast-based-lowering.md plan document (large file, read in sections) 3. Agent provided a terse summary of the 7-phase plan 4. User asked about /delegate command error with "orgin" target 5. Agent fetched copilot CLI documentation 6. Agent explained /delegate doesn't take a target argument and noted the typo 7. User clarified they DO get a remote prompt, pick "origin", and get "Could not determine GitHub repository from remote 'origin'" error 8. Agent ran `git remote -v` to diagnose 9. Agent identified the SSH shorthand `tig:gui-cs/Terminal.Gui` as the problem and suggested fix

Recent Commands: git remote -v was the last command executed]

[Intent Mapping:

1. "Study ./plans. Tersly summarize how you'd go about attacking it." - Review plan docs
2. "the /delegate command is giving me an error when I tell it to target orgin. Why?" - Debug CLI issue
3. Follow-up with actual error message about remote 'origin' parsing failure]

[Technical Inventory: Terminal.Gui project, Markdig AST-based markdown lowering plan, GitHub Copilot CLI /delegate command, Git remote configuration, SSH config aliases]

[Code Archaeology: plans/ast-based-lowering.md - 890-line plan document for refactoring Markdown view]

[Progress Assessment: Both tasks completed - plan summarized, /delegate error diagnosed]

[Context Validation: All critical info captured]

[Recent Commands Analysis: git remote -v showed origin uses SSH shorthand tig:gui-cs/Terminal.Gui which /delegate can't parse]

1. Conversation Overview: - Primary Objectives: (1) "Study ./plans. Tersly summarize how you'd go about attacking it." (2) Debug why `/delegate` command errors when targeting origin remote - Session Context: Two distinct tasks - first reviewing a large refactoring plan, then diagnosing a Copilot CLI issue - User Intent Evolution: Started with codebase planning, shifted to tooling troubleshooting

2. Technical Foundation:

   • Project: Terminal.Gui (gui-cs/Terminal.Gui) - TUI framework for .NET
   • Working Directory: C:\Users\Tig\s\gui-cs\pr-4955-title\
   • Branch context: PR-4955 related to title/markdown work
   • Markdig 0.39.0: Markdown parsing library used in the project
   • GitHub Copilot CLI: User's development tool, /delegate command sends session to cloud agent to create PR

3. Codebase Status:

   • plans/ast-based-lowering.md (890 lines, 37.5KB):
     • Purpose: Comprehensive refactoring plan for Markdown view's lowering pipeline
     • Current State: Plan document, no implementation started
     • Key Content: 7-phase plan (Phase 0-6) to replace regex-based markdown parsing with Markdig AST walking
     • Core Problem: MarkdownPipeline property is dead API — Markdig AST is parsed then discarded, regex re-parses raw text
     • Architecture: Replace LowerFromSourceText() with LowerFromAst(MarkdownDocument) while keeping downstream IntermediateBlock → BuildRenderedLines() → SubView pipeline unchanged
     • Phase 0: Pre-work tests (baseline coverage + known-failure tests)
     • Phase 1: AST block walker (HeadingBlock, ParagraphBlock, ThematicBreakBlock, etc.)
     • Phase 2: AST inline walker (LiteralInline, EmphasisInline, CodeInline, LinkInline, etc.) + add Strikethrough to MarkdownStyleRole
     • Phase 3: Table handling (Option A: extract cell text from AST, feed existing TableData)
     • Phase 4: Code block handling (FencedCodeBlock + indented CodeBlock)
     • Phase 5: The real dragon - nested block flattening (QuoteBlock/ListBlock trees → prefix strings)
     • Phase 6: Wire up, delete regex code, un-skip should-fail tests
     • Files to modify: MarkdownView.Parsing.cs, Markdown.cs, test files
     • Files NOT modified: Layout, Drawing, SubView classes stay unchanged

4. Problem Resolution:

   • Issue: /delegate command fails with "Could not determine GitHub repository from remote 'origin'"
   • Root Cause: Git remote origin uses SSH shorthand tig:gui-cs/Terminal.Gui (custom SSH config alias) instead of standard GitHub URL format
   • Solution Provided: git remote set-url origin git@github.com:gui-cs/Terminal.Gui.git
   • Git remotes found:
     • origin → tig:gui-cs/Terminal.Gui (SSH shorthand - THE PROBLEM)
     • YourRobotOverlord → https://github.com/YourRobotOverlord/Terminal.Gui.git
     • bdisp → https://github.com/BDisp/Terminal.Gui.git

5. Progress Tracking:

   • Completed: Plan review and terse summary delivered
   • Completed: /delegate error diagnosed and fix provided
   • Not Started: Actual implementation of the ast-based-lowering plan
   • User asked "Want me to start implementing?" at end of plan summary — no response yet

6. Active Work State:

   • Current Focus: Diagnosed /delegate error, provided fix
   • Most recent exchange was about the git remote URL for...

Created from Copilot CLI via the copilot delegate command.