Fix sandcastle golden test failures by enabling markdown fenced-code-attributes extension

[Copilot]

The build-linux CI job was failing: sandcastle's internal golden markdown tests (examples, decls) no longer matched their golden files because generated fenced code blocks lost their kanagawa language annotation.

Root cause

• styleCodeBlocks (tools/sandcastle/lib/Sandcastle/DocGen.hs) correctly tags code blocks with the kanagawa class in the Pandoc AST.
• The markdown writer (writePandoc in tools/sandcastle/lib/Sandcastle.hs) enabled Ext_backtick_code_blocks but not Ext_fenced_code_attributes — the extension pandoc requires to actually emit fence info-string attributes (class/language) on output. Without it, the class is silently dropped during rendering even though it's present in the AST.

Fix

• Enable Ext_fenced_code_attributes alongside the other writer extensions in writePandoc:

exts = strictExtensions -- Necessary for raw html
         & enableExtension Ext_tex_math_dollars
         & enableExtension Ext_pipe_tables
         & enableExtension Ext_markdown_in_html_blocks
         & enableExtension Ext_backtick_code_blocks
         & enableExtension Ext_fenced_code_attributes

Repo hygiene

• Added dist-newstyle to tools/sandcastle/.gitignore and removed build artifacts that were accidentally committed while reproducing the failure locally.