feat: JSON-LD hardware reports, a neofetch-style summary, and four missing system entities

`simon snapshot --format json-ld` emits linked data. The existing JSON says
what the readings are; it does not say what any of the words mean, so an
agent has to be told out of band that `percent` is a unit and that
`unavailable` is not a value. The `@context` resolves simon's terms to IRIs
and its units to QUDT, and a consumer that has never seen simon can follow it.

Units map to QUDT where a mapping exists and not where it does not:
`identifier` and `text` are not quantities and carry simon's own term. A wrong
`@type` is worse than an absent one — it is a machine-readable claim that a
GUID is a measurable amount of something, and something downstream will do
arithmetic on it.

Absence survives the encoding: an unavailable reading is a node with no value
and a `simon:unavailableReason`, never a zero, a null, or an omitted node.

`simon fetch` is the neofetch-style summary. It reads through the same
resolver as `snapshot`, so the two cannot disagree about one machine, and it
prints the reason for anything unreadable rather than a dash. "No GPU
detected" and "GPU enumeration failed" call for different responses, and this
is the surface people paste into issues.

Writing it exposed that `system.hostname`, `system.os.version`,
`system.kernel.version` and `system.architecture` were read by `os_info` and
named by nothing. A reader holding data the schema does not expose is
invisible to every agent — the same silence this crate exists to prevent, one
level up. All four are declared and resolved.

Also fixes a real defect in my own JSON-LD: the `@context` defined `unit`
twice, once as the QUDT prefix and once as the property, so the prefix was
silently lost and every `unit:PERCENT` in the graph was an unresolvable name.
The document looked correct and said nothing. The property is now `hasUnit`
with `@type: @id`, and a new test walks the graph checking that every
prefixed name has a defined prefix — the original test checked that terms
were present, not that the names they produce can be followed.

817 lib tests, and the conformance suites green.

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