Add core RelaySelector

[bc1cindy]

#1586 select_relay axis

move OHTTP-relay choice into the payjoin crate as a minimal primitive now (breaking), with a hinge so the AS-aware selection of #919/ #1514 plugs in additively in 1.1 (non-breaking).

integrations diverges today (#1586 (comment))

wallet	relay selection now	converges?
RelaySelector	shared selection fn over the caller's list: uniform-random over non-failed + relay-scoped failover	target
Liana	single relay (user-config)	❌ (list, 1 relay, no rotation)
payjoin-cli	random at bootstrap, cached per session (RelayManager)	❌ (fixable)
ldk-node	user list, walk from random start (relay_order)	❌ (fixable)
Cake	built-in 3-list, random per poll	❌ (fixable)
BBM	same 3-list, shuffle once, in-order	❌ (fixable)
Boltz	single relay (hardcoded)	❌ (list, hardcoded)

target is the shared select() call: today the body is uniform-random over non-failed relays; #919/#1514 fills SelectContext (key-derived, AS-aware, POST/POLL split, ~30s windows, self-AS, dir-AS, role) additively in 1.1, so the call site never changes. failure-tracking stays.

proposed API:

// payjoin::relay
#[non_exhaustive]
pub struct SelectContext {}            // empty = uniform random; #919 adds 
impl SelectContext { pub fn random() -> Self }

pub struct RelaySelector { /* relays + failed */ }
impl RelaySelector {
       pub fn new(relays: Vec<Url>) -> Self;
       pub fn select<R: Rng>(&self, ctx: &SelectContext, rng: &mut R) -> Option<Url>;
       pub fn mark_failed(&mut self, relay: &Url);
 }

// breaking: was (relay, dir) -> OhttpKeys
fetch_ohttp_keys(relays: &[Url], dir) -> (OhttpKeys, Url)

payjoin-cli drops its RelayManager, select() sig never changes, #919 fills SelectContext (request kind,
time window, receiver key) as private fields, and the asmap data arrives via a new constructor on the selector

part of #1586

sounds good?