Prefetch initial file regions by extension

[RenanGBarreto]

Why

Some virtual file formats need initial bytes available before host or application mounting. .exfat and related package files should receive targeted prefetch without reading complete multi-gigabyte files.

Plan

• Add extension-matching file-region rules.
• Match source extensions and optionally virtual paths.
• Default matching region: offset 0, length 10 MiB, interval 300 seconds.
• Generate bounded NBD reads through the current mapper path.
• Keep rules extensible for format-specific regions discovered later.
• Avoid inventing format semantics until measured against real files.

Acceptance criteria

• .exfat, .ffpkg, and .ffpfsc rules support 10 MiB initial reads.
• Unmatched files are untouched.
• Overlapping ranges coalesce and remain bounded.
• Tests cover matching, limits, duplicate rules, and missing files.