Add the NWC pouch: a bounded spending capability for background use

Step 2 of background wallet-connect. Answering while hal is closed means
signing in a service worker, and service workers cannot read localStorage
where the seed lives — which is a property worth keeping, not working around.
So instead of exposing the wallet, carve off a small pouch:

- its own key branch (account chain 6), never used for ordinary coins, so a
  leaked pouch key cannot sign for chain-3 vtxos, the mailbox or LN preimages
- only the pouch's private keys and its vtxo bytes go to IndexedDB, which the
  service worker can read. The seed never does.
- it holds only what the user puts in it

Threat model, stated in the file rather than implied: same-origin script
access or a stolen unlocked device can read IndexedDB and spend the pouch.
That is the accepted cost of answering while closed, and it is bounded — the
pouch is the blast radius. A user who leaves background NWC off has no pouch.

tools/nwc-pouch-test.js asserts the isolation that matters: the pouch branch
is distinct from every other branch, exported keys match their pouch pubkeys
and cannot control ordinary coins, and no seed material appears in the blob.

Co-Authored-By: Claude Fable 5 <[email protected]>

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