sema: refuse work from buffers that do not start like a push

The discovered VA->PA map is content-scanned and can hand a ring entry's
push VA a false translation; observed: GSP firmware pages read as a
12700-byte method stream whose fills would have been executed into
vidmem. Work now requires the buffer to start with a plausible first
method (INC_METHOD, address 0, sane count that fits the length); releases
still apply from such buffers so a shifted stream is not lost. New
'denied' counter in the sweep stats.

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