feat(client): fluent PrivacyBuilder over the wallet seam (#919)

client.build() opens a fluent builder that appends Strk20Actions and hands them to client.submit.
Token ops live under .with(token): deposit / withdraw / transfer / createOpenNote (an open note is a
transfer of the OPEN sentinel to the user — recipient from the new config.userAddress, so
createOpenNote takes no args). Top level: invoke / invokeWithComputation (call builders resolved at
call time with ${openNoteIds[N]} / ${poolAddress} placeholders the wallet substitutes), submit
(proves + broadcasts) and simulate (fee estimate). Open notes must precede the invoke that references
them (createOpenNote after an invoke throws). PrivacyClientConfig gains userAddress. 7 tests.

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