pocx: render bech32 P2WPKH addresses in user-facing account/signer messages

[JohnnyFFM]

Summary

Operator-facing log lines and JSON-RPC errors that referenced a plot or the effective signer printed the 20-byte hash160 as raw 40-char hex, forcing operators to manually re-encode the value to map it back to the pocx1q… form their wallet / get_assignment / the rest of the chain tooling display. This PR renders those identifiers via EncodeDestination(WitnessV0KeyHash{…}) — same pattern already used in get_assignment (src/pocx/rpc/assignments.cpp:104-108).

Address HRP follows the active chain, so testnet/regtest produce tpocx1… / rpocx1… respectively.

The hex form is still computed and passed to wallet lookup APIs (interfaces::Wallet::haveAccountKey, signPoCXBlock, pocx::mining::HaveAccountKey), so the wallet-check contract is unchanged — only the human-facing strings are reformatted.

Refs PoC-Consortium/bitcoin-pocx#3

Touched messages

src/pocx/rpc/mining.cpp (submit_nonce):

• log "Plot %s has assignment, checking key for effective signer: %s"
• JSONRPCError(RPC_WALLET_UNLOCK_NEEDED) "Wallet holding key for effective signer %s is locked…"
• JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY) "No private key available for effective signer %s (plot: %s)"

src/pocx/mining/scheduler.cpp:

• log "[Scheduler] Trying %zu wallet(s) for effective signer %s"

src/pocx/mining/block_builder.cpp:

• log "[BlockBuilder] Building block for account %s (…)"
• log "[BlockBuilder] Plot: %s, Effective signer: %s at height %d"

src/pocx/mining/wallet_signing.cpp:

• "Account ID: %s -> CKeyID: %s" → "Signing for account %s" (the original printed the same hash twice in different reversed-hex forms; collapsed to one bech32 line)
• log "No ScriptPubKeyMan found that can sign for account %s"

Before / after

Before:

No private key available for effective signer 1234567890abcdef1234567890abcdef12345678 (plot: deadbeef…12345678)

After (mainnet):

No private key available for effective signer pocx1qzg6…<bech32> (plot: pocx1qmm…<bech32>)

(testnet → tpocx1…, regtest → rpocx1….)

Test plan

• Build locally (cmake --build build).
• Functional check on regtest: load a wallet that does not hold the key for a plot's effective signer, call submit_nonce, confirm the error string contains rpocx1… for both identifiers.
• Repeat the locked-wallet path: encrypt the wallet, call submit_nonce for a plot it does hold, confirm the RPC_WALLET_UNLOCK_NEEDED error contains rpocx1….
• Trigger the scheduler / block-builder / wallet-signing log lines (regtest forge cycle) and confirm they show bech32.
• Spot-check on testnet (tpocx1…) and mainnet (pocx1…) if available.