feat(relayer): port PROMPT to wayfinder adapter, remove legacy pipeline

[sadiq1971]

Goal

Port PROMPT bridging from the legacy relayer pipeline (processor + source/destination + reconcile loop) to a wayfinder adapter under the TokenBridge interface introduced in #356, then delete the legacy pipeline. Deferred from #356 to keep the USDCx track unblocked (see epic #361).

Work items

• pkg/relayer/bridges/wayfinder/ — bridge.go (Key, Sources, Step), source_eth.go + source_canton.go (moved from engine/source.go), convert.go (per-token decimals replacing const decimalPlaces = 18)
• Step, deposit: idempotency check -> CreatePendingDeposit -> ProcessDepositAndMint -> completed (single step, as today)
• Step, withdrawal: "" -> eth_submitted -> completed; folding CompleteWithdrawal into a stage fixes the existing gap where the retry path skips the post-submit hook
• Resolve token from the deposit event's token field via config (finally using CantonBridge.sol's existing ethereumToCantonToken mappings)
• Delete engine/processor.go, engine/destination.go, engine/source.go and the reconcile loop; PROMPT config moves fully into the per-token Tokens map
• Migrate/regenerate mocks; existing PROMPT unit + e2e tests pass unchanged

Acceptance

• PROMPT deposits and withdrawals behave identically on devstack (existing e2e green)
• pkg/relayer/engine has no imports of pkg/cantonsdk/bridge or pkg/ethereum contract bindings — all mechanism-specific code lives under bridges/
• Adding a second wayfinder-style token is config-only
• In-flight transfers created by the legacy pipeline complete correctly after the cutover (rows already keyed bridge_key=wayfinder by feat(relayer): TokenBridge adapter foundation (interface, registry, driver) #356's migration)

Depends on

• feat(relayer): TokenBridge adapter foundation (interface, registry, driver) #356 (foundation)