build(wasm): let the mem, crdt and client test targets build and run on wasip1

[EnRaiha]

What

Make the test targets of nodedb-mem, nodedb-crdt and nodedb-client build
and run on wasm32-wasip1.

• nodedb-mem, nodedb-crdt: tokio becomes a
  cfg(not(target_arch = "wasm32")) dev-dependency. nodedb-mem also gates
  fluxbench, which declares its own normal tokio = { features = ["full"] }.
• nodedb-mem/src/budget.rs, nodedb-mem/src/governor/reserve.rs: two tests that
  drive the reserve path from std::thread::spawn are native-only, because this
  target has no threads. Their Arc, thread, DatabaseId and TenantId
  imports move into the gated function, as the neighbouring budget test already
  does.
• nodedb-client: the native dev-dependency stays, and a second one covers wasm —
  tokio = { version = "1", default-features = false, features = ["rt", "macros", "sync", "time"] }.

Why

The workspace declares tokio = { version = "1", features = ["full"] }, and Cargo
unifies dev-dependency features across the whole cargo test invocation. On this
target tokio refuses anything beyond sync,macros,io-util,rt,time, so a single
dev-dependency of that shape breaks every wasm test target in the build. All three
crates died before reaching a test:

$ RUSTFLAGS="" cargo check --profile ci --target wasm32-wasip1 -p <crate> --tests
error: Only features sync,macros,io-util,rt,time are supported on wasm.
error: could not compile `tokio` (lib) due to 1 previous error
# exit 101, for nodedb-mem, nodedb-crdt and nodedb-client

Workspace inheritance is additive, so { workspace = true, features = ["rt", "macros"] }
does not avoid full; the wasm entry therefore names the version and the features
directly. rt and macros are supported on this target, which is why
nodedb-client keeps a Tokio runtime there and needs no test gating: all of its
tests run on both targets.

The other twelve shared crates have neither tokio nor fluxbench in a
dev-dependency section, so this is the whole set for the shared-crate wasm build.
nodedb-wal has the same shape and is covered by its own change.

How to check it

export CARGO_TARGET_WASM32_WASIP1_RUNNER="wasmtime -W max-wasm-stack=33554432 --dir=."

# both targets, per crate
for c in nodedb-mem nodedb-crdt nodedb-client; do
  cargo test -p $c
  RUSTFLAGS="" cargo test --profile ci --target wasm32-wasip1 -p $c
done

Crate	native	wasm32-wasip1
nodedb-mem	86 passed, 0 failed	83 passed, 0 failed
nodedb-crdt	168 passed, 0 failed	168 passed, 0 failed
nodedb-client	34 passed, 0 failed	34 passed, 0 failed

The three nodedb-mem tests that do not run on this target are accounted for: two
are the thread tests gated here, and one is excluded by an existing
cfg(not(target_arch = "wasm32")) in nodedb-mem/src/arena.rs.

Before the change the same three cargo check --tests invocations exit 101 with
the tokio error above; after it they exit 0 and the suites run.

cargo fmt --all -- --check exits 0; cargo clippy -p <crate> --all-targets -- -D warnings exits 0 natively and for --target wasm32-wasip1.

Scope

• The workspace's tokio = ["full"] is untouched: narrowing it would change every
  native consumer. The narrowing is per crate, at the dev-dependency.
• Native coverage does not shrink: the two gated tests still build and run there,
  and the native counts above are unchanged from before.
• No public API changed. This is the build half of the wasm test work; the test
  gating in nodedb-wal and the docs/wasm.md note remain in their own changes.
• Test targets only. nodedb-client's optional normal tokio still carries the
  workspace's full, so a wasm build with its remote or native feature enabled
  still fails; that is a question about the crate's feature set, not about its
  tests, and it is left alone here.

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[EnRaiha]

Closing. This change is confined to the wasm32/wasip1 path. NodeDB is a server, does not build for either target, and will not support wasm; that work belongs to NodeDB Lite. If some part of this changes behaviour on a supported target, say which part and it returns as its own PR.