net: add Tor mode for outbound discovery

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+# Tor mode
+
+Bitflash can route outbound discovery traffic through a local Tor SOCKS5 proxy.
+This is transport plumbing only: it does not change consensus, wallet keys,
+mining, `.btf` addresses, or the encrypted rendezvous protocol.
+
+## Client mode
+
+Start Tor locally, then start Bitflash with:
+
+```bash
+bitflash -tor
+```
+
+`-tor` is shorthand for:
+
+```bash
+bitflash -socks=127.0.0.1:9050
+```
+
+If Tor listens somewhere else:
+
+```bash
+bitflash -tor=127.0.0.1:9150
+bitflash -tor=[::1]:9050
+```
+
+When Tor mode is enabled:
+
+- outbound Nostr relay dials use Tor;
+- outbound `.btf` rendezvous relay dials use Tor;
+- destination names are sent to Tor as SOCKS5 domain-name CONNECT requests, so
+  the local DNS resolver does not see them;
+- direct `.onion` dials are refused unless a SOCKS5/Tor proxy is enabled;
+- plain HTTP external-IP probes are skipped.
+
+## Onion rendezvous relay
+
+An operator can expose a Bitflash rendezvous relay as a Tor hidden service. The
+node does not create hidden services itself; Tor owns that configuration.
+
+Example `torrc`:
+
+```text
+HiddenServiceDir /var/lib/tor/bitflash-rendezvous/
+HiddenServicePort 8434 127.0.0.1:8434
+```
+
+After restarting Tor, read the generated onion name:
+
+```bash
+sudo cat /var/lib/tor/bitflash-rendezvous/hostname
+```
+
+Then run the relay on localhost/VPS as usual and announce the onion endpoint from
+nodes that use Tor:
+
+```bash
+bitflash -tor -rvrelay=exampleexampleexampleexampleexampleexampleexampleexample.onion:8434
+bitflash -tor -announcerelay=exampleexampleexampleexampleexampleexampleexampleexample.onion:8434
+```
+
+For a pool operator that wants the pool descriptor to point at an onion
+rendezvous relay, use both:
+
+```bash
+bitflash -operator -tor \
+  -rvrelay=exampleexampleexampleexampleexampleexampleexampleexample.onion:8434 \
+  -announcerelay=exampleexampleexampleexampleexampleexampleexampleexample.onion:8434
+```
+
+## Full-node hidden service
+
+The legacy P2P listener still binds to port `8433`. You can expose it through
+Tor manually:
+
+```text
+HiddenServiceDir /var/lib/tor/bitflash-node/
+HiddenServicePort 8433 127.0.0.1:8433
+```
+
+Current automatic peer discovery is `.btf`/rendezvous based, not onion-address
+gossip. Treat direct full-node onion access as an operator/admin tool unless a
+future release adds first-class onion peer advertisements.