types: read the wall clock through one target-split helper

[EnRaiha]

types: read the wall clock through one target-split helper

Why

wasm32-unknown-unknown has no std clock: SystemTime::now() panics with
"time not implemented on this platform". That target is live, not theoretical —
nodedb-array's HLC is what NodeDB-Lite's array::create_put_slice_roundtrip
drives — so every read on a path Lite compiles needs a clock that answers there.

Eighteen production reads existed across the five shared crates, each doing its
own SystemTime::now(). Nothing in the repo compiled for the target either, so
none of them was covered by anything.

What changed

• clock::since_epoch() owns the split: js_sys::Date::now() on
  wasm32-unknown-unknown, std everywhere else, wasm32-wasip1 included.
• Nineteen production reads route through it, each keeping its own
  conversion, saturation and pre-epoch fallback, so native behaviour is
  unchanged. No shared crate reads SystemTime::now() directly any more; the
  one that remains is inside id_gen's test module.
• The browser arm is now compile-checked. Three generations of getrandom
  reach nodedb-types transitively and each refuses the target until its browser
  backend is selected — 0.2 (via aes-gcm) needs js, 0.3 needs wasm_js plus
  the matching cfg, and the direct 0.4 dependency needs wasm_js. With those in
  place cargo check -p nodedb-types --target wasm32-unknown-unknown compiles.
• js-sys is a workspace dependency, referenced as { workspace = true }
  like every other shared dependency in this repo.

The pre-epoch guard

Date.now() returns an f64, and as u64 saturates a negative value to
zero rather than wrapping. Converting before guarding would report Some(0) — a
plausible-looking zero timestamp — where the std arm reports None, and callers
map that absence to an error. The guard lives in duration_from_epoch_millis,
declared for every target so the host can test it, and two tests pin the contract
either side of the epoch.

Exclusions, stated

• No CI job and no schedule. The wasm CI for Lite belongs in the Lite repo;
  NodeDB runs no scheduled CI.
• No WASI changes. Lite targets wasm32-unknown-unknown and never runs the
  WAL, mem, crdt or client crates under WASI.
• No unrelated fixes. The 32-bit msgpack_scan overflow, the codec element
  count and the vector_primary lint are separate PRs, each with its own
  reproduction.

Steps to test

# native: the helper answers, and the clock does not go backwards
cargo test -p nodedb-types --lib clock

# the browser arm compiles (this is what was previously unverifiable)
RUSTFLAGS='--cfg getrandom_wasm_js' \
  cargo check -p nodedb-types --target wasm32-unknown-unknown

Red arm for the guard — delete the millis < 0.0 branch and the test fails with
left: Some(0ns), right: None.

Not proven here

The browser arm is compile-verified, not runtime-verified: nothing in this
repo executes wasm32-unknown-unknown. Runtime coverage belongs in Lite, whose
nodedb-lite-wasm/tests/array.rs already reaches the array HLC that panicked.

Closes #364

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[EnRaiha]

Closing: the target split exists to serve wasm32, which this repository does not support. NodeDB is a server; wasm work belongs to NodeDB Lite. The part with standalone value — one clock helper over the 19 native call sites — returns as its own PR, without the wasm arm.