feat: add /proc and /sys parsers with fixture tests

Parsers take file contents as a string, not a path, so they test
against committed fixtures with no filesystem mocking.

hwmon is globbed by its name file rather than a fixed index, carried
over from the old config: indices drift across kernel reorders and a
stale one silently reads a different chip.

Failure returns nil, never 0. A sensor reading 0 C is legitimate, so 0
cannot double as an error value.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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