net: compiled-in bootstrap seeds, so a first run does not need a relay (#36)

* net: compiled-in bootstrap seeds, so a first run does not need a relay

A node with no cache has only the Nostr relays. If they are down, blocked or
slow, a first run has no way into the network at all -- and that is the one
moment a user has no patience for. Seeds are the floor under that.

A seed entry is an address and an encryption key. Deliberately no meeting node:
the rendezvous relay pairs on the service's public key, and a `.btf` address is
that key, so a client can find a seed by trying the known relays in turn.
Recording which relay a seed was on would rot the moment it failed over, which
is exactly the failure this project spent weeks chasing -- advertising a
rendezvous somebody is not registered at.

Seeds carry no authority. They serve the same signed, self-certifying
descriptors as any peer, so a hostile seed can stall a bootstrap but cannot
forge a peer or feed a false chain.

The compiled list is empty. Which nodes become permanent public bootstrap
targets is a decision for whoever runs the network, not a default, and it is
close to irreversible once binaries are distributed. Empty is valid: discovery
falls back to Nostr exactly as before. /btfseed=ADDRESS:ENCHEX (repeatable)
appends at runtime, for testing and for private networks.

Tested against the live network, clean datadir, no cache, three seeds carrying
no meeting node:

  btfpeers: no cache yet, falling back to seeds
  btfseed: trying 3 seed(s) across 3 relay(s)
  btfseed: reached q5jxyzn...btf via 90.156.222.107:8434
  btfseed: 3 seed(s) answered

* net: add the Almaty bootstrap node to the seed list

Dedicated node, no wallet balance, no mining, running beside the rendezvous
relay on the same host. It exists only to answer a first dial.

Fresh datadir, no flags, compiled list only:

  btfpeers: no cache yet, falling back to seeds
  btfseed: trying 1 seed(s) across 3 relay(s)
  btfseed: reached ygffz37...btf via 90.156.222.107:8434
  btfseed: 1 seed(s) answered
  btfpeers: kept 2 of 2 descriptor(s)

5.2 seconds from nothing to a peer, against 36s for a cold start that had to go
through the relays. The seed then handed over two more peers by exchange, which
the cache keeps for next time.

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