Pipe table alignment markers (#5) (#46)

* Implement table captions in tree-sitter grammar

Implements support for Pandoc-style table captions that appear after
pipe tables using the tree-sitter grammar for precise parsing.

Table caption syntax:
- Blank line followed by ": caption text"
- Caption must appear immediately after a pipe table
- Only the ': caption' format is supported

Implementation approach:
- Added table_caption rule to tree-sitter grammar as child of pipe_table
- Table captions are parsed at grammar level, not in postprocessor
- Reverted _line rule to disallow paragraphs starting with ':'
  (restores safety check for detecting accidental fenced div continuation)

Changes:
- Modified tree-sitter-markdown/grammar.js to add table_caption rule
- Updated pipe_table processor to extract caption from parse tree
- Added table_caption handler in treesitter.rs dispatcher
- Removed postprocessor caption detection logic and with_blocks filter
- Removed extract_table_caption function from postprocess.rs

Benefits:
- Table captions only recognized in correct context (after pipe tables)
- Restored paragraph safety check prevents malformed documents
- Cleaner separation between grammar structure and AST transformation
- Better performance by eliminating block scanning in postprocessor

Output matches Pandoc's JSON AST structure with caption attached to
table's caption.long field as a Plain block.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

* tests for pipe table alignment

---------

Co-authored-by: Claude <[email protected]>

Author: cscheid

crates/quarto-markdown-pandoc/tests/pandoc-match-corpus/markdown/059.qmd

@@ -0,0 +1,4 @@
+| Right | Left | Center | Default |
+|------:|:-----|:------:|---------|
+| R1    | L1   | C1     | D1      |
+| R2    | L2   | C2     | D2      |

crates/quarto-markdown-pandoc/tests/roundtrip_tests/qmd-json-qmd/table-alignment.qmd

@@ -0,0 +1,4 @@
+| Right | Left | Center | Default |
+|------:|:-----|:------:|---------|
+| R1    | L1   | C1     | D1      |
+| R2    | L2   | C2     | D2      |

crates/quarto-markdown-pandoc/tests/snapshots/json/table-alignment.qmd

@@ -0,0 +1,4 @@
+| Right | Left | Center | Default |
+|------:|:-----|:------:|---------|
+| R1    | L1   | C1     | D1      |
+| R2    | L2   | C2     | D2      |

crates/quarto-markdown-pandoc/tests/snapshots/json/table-alignment.qmd.snapshot

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