feat(client): Strk20Prover seam + core prover + passphrase viewing key (#916)

Strk20Prover (strk20-specific): partialCommitment(dappName) + prove(actions, simulate?). It owns
viewing-key retrieval — never dapp-passed — so a future on-device prover can fetch the key locally.
CorePrivateTransfersProver is the default: derives the viewing key from a passphrase (salted with
the account address, iterated), wraps core PrivateTransfers, and translates Strk20Action[] via its
builder (deposit/withdraw/transfer/OPEN-note/invoke/computeAndInvoke), substituting the
${openNoteIds[N]}/${poolAddress} placeholders against the compiled tx, then maps core CallAndProof
to the strk20 shape. Loads the note registry before proving, saves it after a real proof; simulate
estimates via the node provider. deriveViewingKey + passphraseViewingKeyProvider (lazy, memoized).
PrivacyStorage added to the seam. 20 tests.

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