distd: handlers run on a 4-worker pool; writes serialize (PLAN-HTTPZ A2)

thread_pool_count = 4 turns on std.http's pooled dispatch: reads
(store load + render per request, fully independent) now run in
parallel across workers. POST/PUT stay under one mutex — every write
is a whole-model load-modify-save on dist.db, so concurrent writers
would lose updates; the old sequential loop serialized them by
accident, this serializes them on purpose.

src/server/distd.sx

@@ -1015,6 +1015,11 @@ route :: (store_dir: string, req: *http.Request, resp: *http.Response) {
 // Per-request allocations land in std.http's per-dispatch arena.
 DistdCtx :: struct {
     store_dir: string;
+    // Writes serialize: every POST/PUT is a whole-model load-modify-save
+    // on dist.db, so two concurrent writers would lose updates. Reads
+    // run concurrently across the pool (per-request loads are
+    // independent; SQLite's busy_timeout covers reader/writer overlap).
+    write_mu: thread.Mutex = .{};
 }
 
 distd_handle :: (req: *http.Request, resp: *http.Response, ctx: usize) {
@@ -1030,7 +1035,10 @@ distd_handle :: (req: *http.Request, resp: *http.Response, ctx: usize) {
         }
     }
     if !refused {
+        is_write := req.method == "POST" or req.method == "PUT";
+        if is_write { dctx.write_mu.lock(); }
         route(store_dir, req, resp);
+        if is_write { dctx.write_mu.unlock(); }
     }
     line := concat("distd: ", concat(req.method, concat(" ", concat(req.path, concat(" -> ", concat(int_to_string(resp.status), "\n"))))));
     slog(line);
@@ -1042,6 +1050,7 @@ distd_handle :: (req: *http.Request, resp: *http.Response, ctx: usize) {
 // can't be opened.
 run_server :: (store_dir: string, port: i64) -> !ServeErr {
     dctx : DistdCtx = .{ store_dir = store_dir };
+    if !dctx.write_mu.setup() { raise error.Bind; }
     cfg : http.Config = .{
         port = port,
         max_conn = 256,
@@ -1049,6 +1058,7 @@ run_server :: (store_dir: string, port: i64) -> !ServeErr {
         timeout_request_ms = 120000,   // a large upload must complete within this
         timeout_keepalive_ms = 5000,
         request_count = 200,
+        thread_pool_count = 4,         // reads in parallel; writes serialize above
     };
     srv, se := http.Server.init(cfg, distd_handle, xx @dctx);
     if se { raise error.Bind; }