Rewrite document around flat time (no grids), blocks, and ticks

Alternative:

enum class CreateBlock {
    Create,
    ExtendAdjacent,
    ExtendAlways,
};

or ExtendBlockMode?

TODO add MovementConfig { wrap_across_end?

---

struct Row { Variant<RowInBeat(u16), BeatInMeasure(u16), Measure>?

• format_hex_3 and format_hex_dyn are not used, remove?
• Zooming out all the way from 1/8 triplet should go to 1/4 flat.
• How to handle truncating out-of-bounds pattern notes? When shrinking patterns, do we erase OOB events or not? see #132
• asan

See #113.

Closes #122 (closed).