Fee bump with `fee_absolute` accepts equal fee and does not check fee rate

**Describe the bug**

When bumping a fee with `TxBuilder::fee_absolute`, `Wallet::create_tx` only checks `fee < previous_fee.absolute` (`src/wallet/mod.rs`, `FeePolicy::FeeAmount` arm). Two things are missing compared to the `FeePolicy::FeeRate` arm:

- A replacement paying exactly the same absolute fee as the original passes the check.
- The resulting fee rate is never compared to the original's. A replacement that is larger than the original (e.g. more inputs after coin selection) can end up with a lower fee rate than the original, and even a larger fee may not cover the incremental relay fee.

Such transactions are built without error but are rejected by BIP125 replacement policy (rule 4: must pay for its own bandwidth at the incremental relay fee; Bitcoin Core additionally requires a higher fee rate than the original).

This issue was found by AI.

**To Reproduce**

```rust
let (mut wallet, _) = get_funded_wallet_wpkh();
let addr = wallet.next_unused_address(KeychainKind::External);
let mut builder = wallet.build_tx();
builder.add_recipient(addr.script_pubkey(), Amount::from_sat(25_000));
let psbt = builder.finish().unwrap();
let tx = psbt.extract_tx().unwrap();
let original_fee = wallet.calculate_fee(&tx).unwrap();
let txid = tx.compute_txid();
insert_tx(&mut wallet, tx);

let mut builder = wallet.build_fee_bump(txid).unwrap();
builder.fee_absolute(original_fee);
// Succeeds although the replacement pays no more than the original.
builder.finish().unwrap();
```

**Expected behavior**

`fee_absolute` fee bumps should be validated consistently with the `fee_rate` path so that the builder rejects replacements that would not be relayed.

**Build environment**
- BDK tag/commit: `6fc68462` (master)

**Which backend(s) are relevant (if any)?**
- [x] None / not backend-related (e.g. `bdk_chain`, `bdk_core`)

**Is this blocking production use?**
- [x] No

Start in src/wallet/mod.rs at the FeePolicy::FeeAmount arm in Wallet::create_tx, then compare its checks with the FeePolicy::FeeRate arm. Reproduce the equal-fee case from the issue and add coverage for equal fees and replacements with an insufficient fee rate. Done means fee_absolute rejects replacements that would not satisfy the stated replacement-policy checks.

Written by the indexing model from the issue text.