lwk_wasm: no way to read TxOutSecrets for a PSET output blinded by the wallet

## Context

In a caller-funded PSET flow, a counterparty builds a PSET template containing one foreign input (an atomic-swap claim script) plus a payout output to our wallet. Our wallet then adds L-BTC inputs to cover fees, blinds the PSET, and signs only its own inputs. The counterparty's finalizer has to verify that the payout output really pays the expected asset and amount before it adds the claim witness — which means it needs the unblinded `{ asset, value, assetBlindingFactor, valueBlindingFactor }` for that one output.

## Problem

Once the wallet has blinded the PSET, there is no way to read those secrets back out of `lwk_wasm`:

- `PsetOutput` exposes only `scriptPubkey()`, `amount()`, `asset()`, `blinderIndex()` (`lwk_wasm/src/pset.rs`). `amount()`/`asset()` are the *explicit* values, so they are `undefined` for a blinded output.
- `PsetDetails` exposes balance, signatures, fingerprints and issuances, but nothing per-output (`lwk_wasm/src/pset_details.rs`).
- `TxOutSecrets` already exists with `assetBlindingFactor()` and `valueBlindingFactor()` (`lwk_wasm/src/blockdata/tx_out_secrets.rs`), and `WalletTxOut.unblinded()` returns one — but only for an output already scanned from a confirmed transaction, which is too late here.

The blinder necessarily computes these factors and then discards them.

## Proposal

Expose them at the PSET stage, either:

- `PsetOutput.unblinded(): TxOutSecrets | undefined`, populated for outputs blinded by this wallet, or
- have the blinding call return the per-output secrets alongside the blinded PSET.

Either shape would let a caller hand a counterparty proof of what a blinded output pays without leaking the wallet's master blinding key.

## Versions

Checked against master `1e7332e` and published `lwk_wasm` / `lwk_node` 0.18.0.

Read lwk_wasm/src/pset.rs, lwk_wasm/src/pset_details.rs, and lwk_wasm/src/blockdata/tx_out_secrets.rs, then trace the PSET blinding entry point. Define how per-output secrets should be exposed for outputs blinded by the wallet, while preserving the existing TxOutSecrets shape. Done means a caller can retrieve the relevant secrets from the blinded PSET flow without access to the wallet's master blinding key.

Written by the indexing model from the issue text.