milestone 6: dns cache, rate limiting, query logging, disk monitoring and retention

specs/research/zig-0.16-api-notes.md

@@ -195,3 +195,25 @@ verification name is an IP literal (DoT `tls://1.1.1.1:853`) always fails with
 as an iPAddress SAN — which is how Cloudflare and Quad9 issue theirs. A DoT
 upstream therefore needs a DNS `tls_name` for SNI + verification while dialing
 the IP; verifying by bare IP cannot work on stock 0.16.
+
+## Phase 6 verifications (concurrency, disk, files)
+
+- `std.Io.Queue(Elem)` exists (Io.zig:2184): bounded ring buffer over a caller-supplied array.
+  `put(q, io, elems, min)` with `min = 0` never blocks and returns 0 when full (Io.zig:2218);
+  `getOne` blocks until an item or `error.Closed` after `close(io)`. Elements are copied as raw
+  bytes (`@ptrCast`, Io.zig:2189) — an element holding a slice transfers only the pointer, so
+  queue elements must be self-contained values.
+- `std.Io.Condition` has NO `timedWait` (Io.zig:1653 — wait/waitUncancelable/signal/broadcast
+  only). The timed primitive is `std.Io.Event.waitTimeout` (Io.zig:1827).
+- Disk free space: std has NO statvfs/statfs wrapper (no Statfs struct, no `f_bavail` anywhere in
+  lib/std). Options: `extern fn statvfs` against libc (we always link libc for sqlite) or raw
+  `std.os.linux.syscall2(.statfs, ...)` with a hand-written struct.
+- File append mode does not exist on `Dir.OpenFileOptions`/`CreateFileOptions`. Append pattern:
+  `openFile(io, path, .{ .mode = .write_only })`, `file.writer(io, &buf)`, then
+  `w.seekTo(try file.length(io))`; the writer is positional. `File.setLength` truncates;
+  `Dir.rename` + `Dir.deleteFile` rotate. Positional writes do not serialize — one task owns the
+  log writer.
+- No fixed-capacity hash map in std. Bounded cache shape: fixed slot array plus
+  `StringHashMapUnmanaged(u32)` from key to slot index (`ensureTotalCapacity` once at init);
+  a CLOCK hand walks the stable slot array, never the map. Any map modification invalidates
+  live iterators (hash_map.zig:496).