fix(chain): consider `first_seen` on level ordering #280

Job	Run time
Rust fmt	15s
prepare	3s
Check no_std	36s
Check WASM	2m 52s
Rust clippy	1m 23s
Build & Test (1.88.0, true, --no-default-features --features miniscript/no-std,bdk_chain/hashbrown)	2m 2s
Build & Test (1.88.0, true, --all-features)	4m 20s
Build & Test (1.63.0, --no-default-features --features miniscript/no-std,bdk_chain/hashbrown)	7m 45s
Build & Test (1.75.0, --no-default-features --features miniscript/no-std,bdk_chain/hashbrown)	2m 20s
Build & Test (1.63.0, --all-features)	9m 3s
Build & Test Examples (example_bitcoind_rpc_polling)	40s
Build & Test (1.75.0, --all-features)	4m 47s
Build & Test Examples (example_esplora)	1m 1s
Build & Test Examples (example_cli)	35s
Build & Test Examples (example_electrum)	1m 3s
	38m 45s

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