fix: emit canonical Paykit requests and retry initial links

[ben-kaufman]

Summary

• emit lowercase btc Paykit assets while preserving Locks’ uppercase BTC criterion boundary
• select the Bitcoin endpoint identifier from the configured network
• encode private Bitcoin endpoints using the canonical { "value": ... } payload expected by wallet adapters
• retry link_establishment every second during a bounded initial handshake window before returning to normal exponential backoff
• wake the durable outbox loop at the selected retry delay without changing idempotency, fencing, or no-public-fallback semantics
• update the local reader demo and Paykit rc43 dependency

Verification

• cargo fmt --all -- --check
• cargo check --workspace
• cargo test -p paykit-server link_establishment_retries_rapidly_before_restarting_exponential_backoff
• cargo clippy --workspace --all-targets -- -D warnings
• focused create-invoice, reader-demo, and outbox suites from the canonical request fix
• clean Locks → Paykit Server → Bitkit proof is being rerun for iOS and Android

Found while running the clean Locks → Paykit Server → Bitkit E2E flow.

[ben-kaufman]

@jvsena42 could you please review the current head? The rapid initial private-link retry, canonical endpoint changes, and regression coverage have all been exercised in the clean iOS and Android Locks → Paykit → Bitkit E2E flow.

[ovitrif]

Approved. Canonical lowercase btc assets, network-specific endpoint IDs, {value} private payloads, and the bounded rapid link_establishment retry preserve the intended Paykit request and outbox behavior.