perf(sync-service): parallelize Aura call proofs during parachain bootstrap

[replghost]

Summary

During parachain bootstrap (bootstrap_parachain_consensus), the two Aura call proofs are fetched sequentially:

1. Download AuraApi_slot_duration call proof → execute
2. Download AuraApi_authorities call proof → execute

These are independent of each other. Fire both downloads in parallel with join, then execute sequentially. Saves one P2P round-trip on every start (cold and warm).

Context

Part of a series of architectural improvements to parachain startup time. See also:

• fix: parachain warm-restart hang #3210 — fix parachain warm-start hang
• perf(runtime-service): use short retry when no peers available #3213 — exponential backoff for no-peers retry
• perf(sync-service): skip parachain runtime download on warm start #3214 — cache runtime code in DB

Effort

Trivial — change two sequential awaits to a join.

[replghost]

Starting work on this issue.

[replghost]

PR opened: #3223

[replghost]

PR opened: #3225 — this PR parallelizes all three downloads, subsuming #3219.