fix: resilient worker accept loop — survive client disconnects

[cjchanh]

Summary

The worker accept loop (Worker::run) silently exits on any accept() error, permanently killing the TCP listener. This causes a "one connection works, then dead worker" failure mode.

On iOS this is particularly severe: the first master disconnect (broken pipe, network hiccup, master restart) leaves the worker completely unreachable until the app is force-killed and relaunched.

Changes

• Hold TcpListener in Option for in-place rebinding
• Classify transient errors (ECONNABORTED, EINTR, TimedOut, WouldBlock) and retry immediately
• On fatal accept errors, drop and rebind the listener on the same port without reloading model weights
• Log listener fd and local address before each accept cycle for diagnostics
• Add describe_listener() helper for cross-platform fd reporting

Testing

• cargo test -p cake-core --lib — 589 passed, 0 failed
• Live iOS worker (iPad Air M3, iPadOS 26.2): 20 sequential inference requests over 6 minutes with zero drops or reconnections needed
• Before this fix: worker died after first master disconnect, every time

Diagnostic log lines (after fix)

worker accept loop awaiting master on 0.0.0.0:10128 fd=9
[10.0.0.203:52341] connection loop ended: broken pipe
worker accept loop awaiting master on 0.0.0.0:10128 fd=9   ← re-enters accept

Recovery path (fatal accept error):

accept failed on 0.0.0.0:10128; dropping listener and rebinding ...
worker listener bound on 0.0.0.0:10128 fd=12

Fixes #79