Page variables and $file.path

Acceptance Criteria

• $page.path is exposed, replacing $page.filePath; POSIX-normalized (forward slashes regardless of host OS)
• $page.filePath is removed (not aliased)
• Changeset documents the rename as a breaking change
• $page.dir resolves to the directory portion of $page.path; empty string for content-root pages
• $page.dir is POSIX-normalized and has no trailing slash
• $page.slug resolves to the last URL segment of $page.url
• $page.slug for index pages resolves to the directory name (not "index")
• $page.title resolves to $frontmatter.title when present and non-empty after trimming whitespace
• $page.title falls back to the first H1 in the AST when $frontmatter.title is absent, empty, or whitespace-only
• $page.title's H1 walk is depth-first and descends into tag (rune) children; the first markdown heading node with level: 1 wins
• $page.title is undefined when neither source provides a title
• $file.path resolves to the source file's project-root-relative path, POSIX-normalized
• $file.created, $file.modified, $page.url, $page.draft, $frontmatter.* continue to work (no regression)
• Project root (config-file directory) is threaded into processContentTree for $file.path computation
• Variables work in attribute interpolation (path=$page.path), text interpolation ({% $page.title %}), and conditional tags ({% if equals($page.dir, "docs") %})
• Partials and layouts included into a page see the host page's $page.* / $file.*, not their own file's
• Test fixture covers each new variable
• Authoring docs page documents the full public variable surface ($frontmatter.*, $page.*, $file.*) with examples; explicitly explains the page-vs-file frame distinction and the __ prefix convention