Warning, after I wrote this I have obtained the actual documentation

c2h format

divided in sections:
header has name (4 chars), followed by length (4 bytes), not including header

CC2H: contains "7", version number. at start of file,
then follows multiple (FILE, TYPE, NAME, SAVE), one set for each level,
then "END " (padded with space) at end of file

FILE: filename containing level. NOT used for loading the file, see SAVE
      offset 0x00
TYPE: name of levelset, can be empty string
NAME: level name
SAVE: 100 bytes of score info and other stuff

info about SAVE:

offset	meaning
0x00	line number into .c2g file. used for fetching the filename for the
	level and music file. if line doesn't contain this, the game tries the
	next line(s). adding/removing lines from the .c2g file can result in
	the wrong level being loaded when choosing "Select level" from an
	existing score file
0x04	some kind of total score, possibly the actual total score when the SAVE
	section was (re)written, which must happen more often than score/time
	improvement (pressing F6 to recalculate a wrong line number does not
	write the current total score to this offset)
0x08	highest bonus score achieved (does not include bonus from time left)
0x0c	always 0 for untimed levels, has a value around 50-60 times time left
	on timed levels. it's not (time left)*60 (frames per second), but
	somewhat close. really no idea
0x10	no idea, it's non-zero for very few levels (COBBLER=119, AVALANCHE=10,
	THINK FAST=25, CRAZY=20 and some more)
0x14	level bonus, (level number)*500
0x18	level number
0x1c	no idea, but always 0
0x20	no idea. most common value is 1, but i have up to 30 in my file. it
	can't be something like number of plays or wins, doesn't match what i
	would have (1 on levels i definitely won more times, i imagine my max
	would be higher)
0x24	no idea, 22 for levels 2 4 5, otherwise 0 in my old score file. a fresh
	savegame has 22 for all levels in the base game... i've seen 86 in
	custom sets
0x28	no idea, but always 0
0x2c	no idea, 1 for levels 2 4 5, otherwise 0. in a fresh save file 2
	occurs
0x30	1=level solved, 0=unsolved
0x34	no idea, always 0
0x38	no idea, always 0
0x3c	no idea, always 0
0x40	no idea, always 0
0x44	no idea, always 0
0x48	no idea, always 0
0x4c	no idea, always 0
0x50	no idea, but it seems this and the next couple of values are 1 byte.
	first levels with non-zero values (for 0x50-0x53) are 11 14 20 21 22.
	identical values in old and fresh savegame
0x51	no idea
0x52	no idea
0x53	no idea
0x54	no idea
0x55	no idea
0x56	no idea
0x57	no idea
0x58	highest level time achieved
0x5c	highest level score achieved
0x60	some kind of checksum? does not match any variant of crc-32 from
	http://crccalc.com/, with just data, length+data and SAVE+length+data.
	then i noticed several levels with the same value (6,7 and
	37,38,39,40), strongly indicating something else than a checksum