Make default Markdown publication atomic and root-contained

[chubes4]

Problem

The default post-type-hierarchy layout is the canonical production layout, but WP_Markdown_Storage::write_post() publishes with an in-place file_put_contents(..., LOCK_EX). A terminated or failed write can leave the authoritative Markdown file truncated or partial.

This legacy path also creates and traverses directories without applying the symlink-segment and root-containment checks used by custom layout profiles. A linked segment inside the canonical tree can redirect publication outside MARKDOWN_DB_CONTENT_DIR.

Relevant implementation: inc/class-wp-markdown-storage.php, default-layout write path around lines 276-308. The default profile is marked legacy in inc/class-wp-markdown-content-layout-profiles.php.

This is a concrete storage-integrity prerequisite for the pure-PHP canonical engine tracked in #232 and may also close one class of durability failure represented by #99.

Required outcome

• Publish default-layout Markdown through a same-directory staged file and atomic replacement.
• Preserve the previous complete canonical file whenever staging or publication fails.
• Validate every destination path segment beneath the canonical root and reject symlinks or directory replacement.
• Retain current hierarchy promotion, stale-path cleanup, indexing, and mutation receipts.
• Add deterministic coverage for failed publication, interrupted-write safety, and linked path segments.

Acceptance criteria

• Readers observe either the previous complete file or the next complete file, never a partial write.
• A failed replacement leaves the prior canonical post readable and unchanged.
• No default-layout write can escape the configured content root through a symlinked segment.
• Existing default-layout and parent-promotion tests continue to pass.

AI assistance

GPT-5.6 Sol via OpenCode reviewed the storage implementation, identified the publication and containment gaps, and helped structure this issue and its acceptance criteria. Chris Huber directed the review and issue creation.