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rbplus: add the REFLEC BEAT plus group

Three commands. unpack converts a whole download to formats that open
outside iOS, extract-assets unpacks one of the downloadable texture
archives, and dump-chart writes one note chart as JSON and optionally
draws it. Mach-O images are the one thing left behind entirely.

A tune package is a ZIP whose entries use the same BFCodec cipher as
pop'n rhythmin and jubeat plus under two keys of its own. Neither key
appears in the executable as a passphrase: each is stored with every byte
reduced by its own index, so adding the index back yields the phrase
whose MD5 is the key. A package does not record which of the two it uses,
so the first is tried and the second used when the metadata does not
parse, which is what the game itself does.

The RBFF chart wanted more care. A note's hit time is its two stored
times added. A chain is the note record's own doubly linked list rather
than a grouping by any field, and every one of the five flag bits is
exactly redundant with another field.

What decides a note's lane is its route selector, derived from the second
target coordinate. One naming a lane, 0 to 6, comes straight down into it
and no randomness applies: that is every slide and every vertical note.
One naming 7, 8, or 9 is aimed at one of the three alternative targets
beyond the lanes. Only a note naming nothing is placed by the engine's
tracker, which shuffles with rand() when play starts, so that part of a
chart falls differently every time. Those notes are placed here from a
seed, fresh each run unless --seed pins one.

A slide's records are its waypoints: each gives a lane and, in the same
shape a note's own timing takes, a spawn time and a travel time whose sum
is the moment to be there. The image draws the track between them.

Signed-off-by: Andrew Udvare <audvare@gmail.com>

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