Last iteration ended with 'what reads the flag file is unknown' after an offset
search failed. The framing was wrong: I was hunting an engine-side reader, but
the consumer is the script itself, through built-in 9 (read_freg), which loads
[phase+88][i] into the double result register.
Stage02.ssb calls read_freg 12 times -- the same count as set_flag (12) and
clear_flag (12) -- so the latch is symmetric and entirely inside the VM. That
closes the middle of the set_flag -> ... -> END_PHASE chain: one coroutine
latches, another reads it back and branches.
Also records an unconfirmed observation: op10 + op13 pairs with consecutive
small immediates and their own code offsets look like a case/branch dispatch.
Flagged as a pattern, not named -- neither handler has been read.
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