Vanilla's SECTION_BIOMES palette strategy switches on the bit count with
`tableswitch {0..3}`: 0 single-valued, 1-3 linear, and everything else falls
through to the global palette. There is no hashmap tier for biomes — that exists
only for block states, whose 0..8 switch we already implement correctly.
We were writing a linear palette all the way up to 7 bits. A section holding 9 or
more distinct biomes therefore went out as a 4-bit indirect container while the
client read it as global: no palette prefix consumed, long array re-read at 7
bits, and every field after it in the chunk payload misaligned. Sections that
straddle the surface and the cave biomes really do carry that many, so this is
reachable in ordinary terrain rather than a corner case.
Checked against the jar rather than recalled: javap -c on Strategy$2 shows the
{0..3} switch with Configuration$Global in the default arm.
The test decodes each container the way the client would and requires it to
consume exactly the bytes we produced, so a misframed container shows up as a
byte count instead of needing a client to notice. Restoring the old threshold
fails the 9-, 20- and 65-biome cases.
c399070f59