Centralize retry/backoff configuration on `backon`; inject retries where components give up

[emlautarom1]

Summary

backon is already the workspace retry engine and app/src/retry.rs (the Charon app/retry port) is built on it — but the workspace has drifted into at least 5 distinct backoff parameter sets and 4 drive mechanisms:

• retry.rs defaults (250ms/12s/1.6) — zero call sites (feat(app): wrap duty callbacks with the retry executor #534 tracks wiring it into the duty callbacks).
• core::expbackoff fast() (100ms/5s) and default() (1s/120s) — used by scheduler/sse/bootnode, sometimes driven manually via .next() + expect.
• p2p/src/relay/dial.rs#L104-L126 — a hand-written duplicate of expbackoff::default() (its doc even cites the same Charon config), reimplemented because p2p needs a pollable Duration rather than an async wrapper.
• quic_upgrade.rs — a third scheme in units of minutes (1→512, doubling).
• eth1wrap — Alloy's RetryBackoffLayer::new(10, 1000, 100), a fourth policy in a foreign library's units.
• Fixed-delay loops in dkg/sync (250ms), cli test/peers (for attempt in 0..5, 5s intervals). Same family: dkg busy-polls node signatures on a 100ms ticker, re-locking and cloning the accumulated slot vector every tick (nodesigs.rs#L144-L164), and exchanger.rs#L561 polls with a bare 100ms sleep — both want a Notify/watch signal instead of a poll.

Meanwhile, components that should retry just give up and wait for the next tick: scheduler resolve_duties errors are logged "(retrying next slot)" — a beacon-node blip loses a slot's duty resolution; bcast/recast retries next epoch; the wire-layer store/broadcast sinks log and swallow errors (wire.rs#L739-L746 and siblings) — precisely the points Charon wraps in async-retry.

Proposed change

• One retry module (the existing app::retry + core::expbackoff, merged or clearly layered) exposing the named policies (fast, default, plus a pollable-Duration helper for poll-based behaviours so relay/dial.rs can delete its copy).
• Replace the manual .next()/hand-rolled loops with backon's Retryable or the shared pollable helper; express eth1wrap's policy in the same config vocabulary.
• Inject retries at the give-up sites above (scheduler duty resolution, recast, wire sinks), with per-duty deadlines from the existing DeadlineCalculator plumbing. feat(app): wrap duty callbacks with the retry executor #534 covers the five Charon duty-callback wrap points; this issue covers the config unification and the remaining sites.