Fix serve registration crash on 0.7.61: node_hmac_key is a property, not a method

[shaal]

Fixes #10.

cli.py:1232 (the #456 / TC-9c token/HMAC cache block) calls
node.node_hmac_key(), but node_hmac_key is a @property (node.py:674)
that returns a str. Invoking it raises TypeError: 'str' object is not callable.

Because that call sits inside the registration try/except, the error is
caught and logged as Registration attempt failed; all 3 retries hit the same
deterministic error, and the node never reaches a registered client state. Its
heartbeats then stop refreshing the directory and it is marked offline
(available:false, health:0) within ~30s — even though the process is still
running and the HTTP registration succeeded server-side (token returned,
operator delegation accepted). 0.7.48 is unaffected.

Fix

Read the property instead of calling it:

-                        hmac_key = node.node_hmac_key()
+                        hmac_key = node.node_hmac_key

Test

pipx install --force 'iicp-client[nat]==0.7.61'   # repro: serve logs the crash, node goes offline
# with this patch applied, serve registers cleanly and the node stays available

Follow-up (not in this PR)

This cache step is described in-code as "best-effort", but it runs inside the
registration try/except, so any error here aborts an otherwise-successful
registration. Wrapping just the cache step in its own try/except Exception: pass would make a future caching bug non-fatal to registration.