fix: guard window.acDISK_SEND against duplicate module instance overwrite
When importWithRetry races WS blob URL + HTTP imports, both module
instances execute top-level code. The second overwrites acDISK_SEND
with its own diskSends array while boot() uses the first module's
consumeDiskSends — splitting the queue so session:started never
arrives at disk.mjs.
Guard acDISK_SEND and AC master volume functions to only bind from
the first module instance that loads.