First reported by @ViktorTigerstrom

Attempting to bump litd triggers a WASM build failure. Too many symbols from taproot-assets are included, leading to relocation errors like:

type:...ProofType: non-pc-relative relocation address ... is too big: 0x100d70000
...
type:...SortDirection: non-pc-relative relocation address ... is too big: 0x100e90000

This doesn’t happen before the version bump — likely due to changes in the dependency pulling in more of taproot-assets than needed.

Proposed fix: move taprpc to a submodule in taproot-assets. This lets the WASM client depend only on taprpc, avoiding the full dependency and reducing symbol load.