Change the graphics protocol N key from a boolean into a usage-hints
bitmask. Define the first bit as a transient hint, allowing the terminal
to treat the image data as short-lived and apply optimizations such as
skipping disk cache writes.
Propagate the transient hint through frame coalescing and composition, so
a composed frame is transient if any contributing frame is transient.
Add a new graphics protocol key, N=1, to request that transmitted
image/frame data is kept only in memory and not written to the graphics
disk cache file.
This is useful for transient high-frequency updates such as video-like
streams, where the latest frame is the only useful data and persisting
each frame to the disk cache causes unnecessary write traffic.
The implementation keeps the existing graphics cache abstraction intact:
memory-only entries can still be read back by animation, composition, and
frame coalescing paths. Only persistence to the disk cache file is skipped.
The default behavior is unchanged when N is omitted or set to zero.
