WASM build broken on `litd` version bump due to taproot-assets symbol bloat

[ffranr]

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.

[ffranr]

We probably just need to complete/merge this: lightninglabs/taproot-assets#1090

EDIT: this will be the working PR: lightninglabs/taproot-assets#1487

[ffranr]

This is the solution I think we should implement to fix the WASM build issue caused by importing taproot-assets (which pulls in too many symbols):

• Split taprpc into its own submodule within the taproot-assets repo, allowing it to be imported independently without the full taproot-assets dependency. (DONE: taprpc: extract as standalone Go submodule for WASM clients taproot-assets#1487)
• Update litrpc to depend directly on taprpc. Move necessary code from litclient into litrpc to eliminate another indirect dependency on taproot-assets. (DONE: Refactor litclient and perms to reduce LNC dependencies lightning-terminal#1057)
• Modularize lit/perms so it can be imported separately, without requiring the entire lightning-terminal repo (which depends on taproot-assets). (DONE: Refactor litclient and perms to reduce LNC dependencies lightning-terminal#1057)
• Move loop/perms into the looprpc module to avoid pulling in the full loop repository. (DONE: perms: move perms.go into looprpc loop#930)
• Modularize gbn, mailbox, and hashmailrpc within lightning-node-connect to avoid importing the whole repository and its taproot-assets dependency.

This approach isolates only the necessary RPC and permission modules for wasm-client and avoids bringing in all of tap, allowing the WASM build to succeed.

I've tested this wasm-client build setup with lit at the latest master and it builds without error. Just need to put up the rest of the necessary PRs now.