Phase 4b follow-on: pin --ttl RPC + CLI

Closes Phase 4b by exposing the TTL sidecar written by
Phase 4b primitives on the wire and via `claw-cargo pin`.

Wire additions:
* Method::SetTagExpiry (0x1a) — payload `key_len:u16 || key ||
  expires_at:u64 (LE)`. Reply single-byte OK. `expires_at == 0`
  clears the sidecar.
* Method::GetTagExpiry (0x1b) — payload raw key bytes. Reply 8
  bytes (u64 LE) on hit; NotFound when no sidecar is present.

Both accept writes even when the stamped tag itself is absent,
matching `TagStore::set_stamped_expiry` semantics — the sidecar
takes effect the moment the tag lands.

CLI:
* `claw-cargo pin --ttl <duration>` — humantime-style duration
  (`30d`, `1h30m`, `2w`, ...). Applied to both the primary tag
  and its `.fingerprint` companion so eviction treats them as
  one lifetime. `--ttl 0` / `clear` / `none` clears an existing
  sidecar without touching the value.

Tests: encode/decode roundtrip + malformed-input rejection for
`encode_expiry_record`, method-byte stability, NotConfigured
without a tag store, end-to-end set/get/overwrite/clear over
QUIC, and a real-pin flow that publishes a stamped tag then
attaches TTL. Duration parser is unit-tested for single/compound
forms, case-insensitive units, bad input, and clock alignment.

No new deps — the humantime-style parser is 60 lines in-tree.

Co-Authored-By: Claude Opus 4.7 <[email protected]>

claw-store/src/cluster/rpc/client.rs

@@ -598,6 +598,65 @@ pub async fn call_get_tag_versioned(
     ))
 }
 
+// ── Phase 4b follow-on: TTL client helpers ───────────────────────────
+
+/// Phase 4b follow-on (2026-07-13): attach a TTL sidecar to a stamped
+/// tag on a peer. `expires_at_unix == 0` clears any prior sidecar.
+///
+/// The peer accepts writes even when the stamped tag isn't present
+/// yet — the sidecar sticks around and takes effect once the tag
+/// lands (`TagStore::set_stamped_expiry` semantics).
+pub async fn call_set_tag_expiry(
+    conn: &Connection,
+    key: &str,
+    expires_at_unix: u64,
+) -> Result<()> {
+    let payload = crate::cluster::tags::encode_expiry_record(key, expires_at_unix);
+    let reply = rpc_call(conn, Method::SetTagExpiry, &payload).await?;
+    if reply.len() != 1 {
+        bail!(
+            "expected single-byte SetTagExpiry reply, got {} bytes",
+            reply.len()
+        );
+    }
+    match reply[0] {
+        STREAM_STATUS_OK => Ok(()),
+        code => match decode_error(code) {
+            Some(err) => bail!("peer rejected SetTagExpiry: {}", err.describe()),
+            None => bail!(
+                "peer replied with unknown byte 0x{:02x} for SetTagExpiry",
+                code
+            ),
+        },
+    }
+}
+
+/// Phase 4b follow-on: fetch the TTL sidecar for a stamped tag.
+/// Returns `Ok(None)` when no sidecar is present (never expires or
+/// no such tag).
+pub async fn call_get_tag_expiry(
+    conn: &Connection,
+    key: &str,
+) -> Result<Option<u64>> {
+    let reply = rpc_call(conn, Method::GetTagExpiry, key.as_bytes()).await?;
+    if reply.len() == 1 {
+        match decode_error(reply[0]) {
+            Some(ErrorCode::NotFound) => return Ok(None),
+            Some(err) => bail!("peer replied with error: {}", err.describe()),
+            None => {}
+        }
+    }
+    if reply.len() != 8 {
+        bail!(
+            "expected 8-byte GetTagExpiry reply, got {} bytes",
+            reply.len()
+        );
+    }
+    Ok(Some(u64::from_le_bytes(
+        reply.as_slice().try_into().expect("checked length"),
+    )))
+}
+
 // ── Phase 5g: cache metrics client helper ────────────────────────────
 
 /// Fetch the peer's current cache-metrics snapshot.