Enhance stat-cache-hitrate reports with legends and file:// links (path quoting)

[mouxinqq]

Motivation

• Improve readability of generated cache hitrate reports by adding chart legends and clearer guidance on how to interpret graphs.
• Make generated report file references clickable/usable by providing absolute paths and file:// URIs that are safe for spaces and non-ASCII characters.

Description

• Add Path and urllib.parse.quote imports and implement _build_path_links(path) which returns a resolved absolute path and a percent-encoded file:// URI.
• Extend templates and report formatting in stat_cache_hitrate.py and the Markdown template to include chart legends and brief explanations for Unicode bars, ASCII sparklines, and Q1→Q4 trends.
• When exporting reports, print both the absolute report path and a file:// URI and similarly expose the details/per_window_data.md path if present.
• Small refactors: add inline explanations to format_full_report and format_tail_report, and update report_templates.md to include legend and example URIs.

Testing

• No automated tests were executed for this change.

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Codex Task