fix: improve atomic ordering in ThreadPool and NAPI init (#310)

## Motivation

`ThreadPool` worker hot loops use `.acquire` on `err_flag.load()` where
`.monotonic` suffices — the flag is a pure early-exit signal with no
data dependency on the setter's other writes. BLS verification is
CPU-intensive, so relaxing this in the inner loop avoids unnecessary
memory-fence cost. Matches the pattern already used in
`src/state_transition/cache/pubkey_cache.zig`.

The earlier NAPI init-mutex changes from this branch have been dropped
after merging main, because main moved to zapi-managed lifecycle
(`js.exportModule` with `init`/`cleanup` hooks). The concurrent-register
race they were guarding against has been filed against zapi upstream:
ChainSafe/zapi#31.

## Description

`src/bls/ThreadPool.zig`:
- `err_flag.load(.acquire)` → `.monotonic` in `VerifyMultiWorkItem.exec`
and `AggVerifyWorkItem.exec` worker loops.
- Setter side (`err_flag.store(true, .release)` on pairing failure) is
unchanged — release semantics on the producer side carry no obligation
on the consumer to also be `.acquire` when the consumer doesn't depend
on the producer's other writes.

Author: GrapeBaBa

src/bls/ThreadPool.zig

@@ -203,7 +203,6 @@ const VerifyMultiWorkItem = struct {
         const self: *VerifyMultiWorkItem = @fieldParentPtr("base", base_item);
         const job = self.job;
 
-        // Each worker gets its own pairing buffer on the stack
         var buf: PairingBuf = .{};
         var pairing = Pairing.init(&buf.data, true, job.dst);
 
@@ -213,7 +212,7 @@ const VerifyMultiWorkItem = struct {
         while (true) {
             const i = job.counter.fetchAdd(1, .monotonic);
             if (i >= n_elems) break;
-            if (job.err_flag.load(.acquire)) break;
+            if (job.err_flag.load(.monotonic)) break;
 
             did_work = true;
 
@@ -347,7 +346,7 @@ const AggVerifyWorkItem = struct {
         while (true) {
             const i = job.counter.fetchAdd(1, .monotonic);
             if (i >= job.n_elems) break;
-            if (job.err_flag.load(.acquire)) break;
+            if (job.err_flag.load(.monotonic)) break;
 
             did_work = true;