Grid: the empty-digest quorum guard, and the instrument on the manifest

[JamesKane]

Two AppView-side changes that came out of reviewing the Navigator edge
(decodingus-navigator#61).
Both are guards the AppView needs regardless of how well-behaved a node is.

Two empty digests must not form a quorum

A digest with no comparable call agrees with every other such digest — every field is
absent on both sides. a_shapeless_digest_agrees_with_nothing_real already asserted
exactly that and called it "equally uninformative", and I did not follow the thought
through: two nodes whose analysis failed would agree, canonicalize a unit with no
content, and be credited for it.

digest::has_content now answers whether a digest is about anything, and
grid-validate refuses to canonicalize an empty winning cluster. One discrete call
is enough.

The Navigator side was fixed in the same pass — a node now fails its unit rather than
submitting an empty digest. This guard is here anyway, because a node is untrusted
by construction and that is the whole premise adaptive replication rests on. A check
that only runs on the honest side is not a check.

The instrument on the curation manifest

A node cannot choose a mapper preset without it. A read set with no mate is either a
long read or a single-end short read, and those need different presets — a long read
mapped under a short-read preset does not fail, it produces alignments that look
right and are wrong.

crawl_project already stores ENA's instrument model on the library, so this
publishes a value we already hold. Navigator feeds it to Preset::infer, which errors
rather than guesses on an unrecognised instrument, so the unit goes back to the pool
instead of a result of unknown quality reaching the quorum.

Without this, every single-end Illumina run in the catalogue would have been mapped as
HiFi.

Verifying

du-db's grid tests are live-Postgres and skip without DATABASE_URL. To run them:

PGHOST=$(container inspect du-testpg | python3 -c "import sys,json;print(json.load(sys.stdin)[0]['networks'][0]['ipv4Address'].split('/')[0])")
DATABASE_URL="postgres://postgres:dev@$PGHOST:5432/postgres?sslmode=disable" \
  cargo test -p du-db --test grid -- --test-threads=1

Serially, with ≥4 GB — each test migrates its own database.

CI

origin/main still fails cargo clippy --workspace -- -D warnings under current
stable clippy, in files this branch does not touch (du-db/src/place.rs:297
is_none_or vs the declared MSRV 1.80, plus du-web/src/origins_layout.rs:505,512).
Raised on #32 and still unaddressed; happy to fix in a separate PR if you want it.

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