itest: cover custom-anchor MuSig2 key-path and tapscript script-path spends

[darioAnongba]

Closes #161.

Adds integration coverage for the two custom-anchor signing surfaces that the existing custom-anchor itests deliberately assert are empty (require.Empty(signingRequests.MuSig2) / require.Empty(signingRequests.ScriptPath)). Both are Ark-agnostic — described purely in builder terms (tapscript leaf / MuSig2 anchor key, script-path / key-path spend, proof verification).

TestCustomAnchorBuilderScriptPathTimeoutSweep

Unilateral timeout exit:

1. Locks a minted asset into an OP_TRUE output whose Bitcoin anchor commits a CSV relative-timelock tapscript leaf as a sibling to the asset commitment, under a NUMS internal key (so the timeout leaf is the only spending path).
2. Matures the timelock, then unilaterally sweeps through the script path: the asset input is authorized by the regenerated OP_TRUE witness (zero signatures, no counterparty), and the anchor is spent by signing the timeout leaf with a single key. The control block is reconstructed from the committed TaprootAssetRoot/TaprootMerkleRoot, exactly as an integrating host composes it.
3. Verifies the swept proof imports and the balance transfers — the asset chains correctly and is not burned.

Closes the "proof regenerated without cooperation" property.

TestCustomAnchorBuilderMuSig2KeyPathSweep

Cooperative exit:

1. Locks a minted asset into an OP_TRUE output anchored to a MuSig2-aggregated internal key (AggregateKeys(participants, false).PreTweakedKey, matching how the SDK reconstructs the committed key).
2. Cooperatively sweeps it with a single aggregate key-path signature produced by an in-process two-party MuSig2 ceremony (nonce aggregation, partial signing, BIP-341 taproot-tweaked combine), applied via ApplyKeyPathSignature.
3. Verifies the swept proof imports and the balance transfers.

Notes

• Both sweeps are caller-funded and broadcast directly by tapd (no external fee-bump child), which keeps them independent of shared miner-wallet state.
• Ark VTXO-tree orchestration, connectors, forfeits, and arklib closures remain out of scope — they build on these primitives in the SwapDK repos.

Verified against the pinned tapd itest stack, including the full custom-anchor suite in one process (CI ordering): all four custom-anchor itests pass. go build ./..., go vet ./..., -tags itest build/vet, and gofmt all clean.