iyi 0.9.0

The collector races Go, and is measured against it. 0.8.0 seated iyi's
own collector as the default; this release gives it the rest of its design
and a number beside the collector it set out to match. The runtime has a
kernel thread now — raw clone and futex on Linux, libSystem's pthreads on
darwin, a stop-the-world by signal that reads the stopped thread's registers
out of its context, and a thread-local that is the language's own
@[ThreadLocal] — measured against the dependency floor before it was
designed, and the floor did not move. On it: a parallel mark on helper
threads with a stack per worker and a pool of batches; a mark that runs
beside the program on a write barrier the compiler emits, a collection being
two stops of tens of microseconds where it was one of milliseconds, with
Go's two answers to a mutator outrunning the mark — allocate black, and
assist; a sweep that runs beside the program too, in slices of an arena
taken by the helpers after every collection and by an allocating thread only
for the slice it needs; and a footprint that follows the live set — a
one-word object header, eight-byte size classes, pages handed back to the
kernel by the sweep and taken up again by the carve, and IyiMark.growth,
Go's GOGC with Go's meaning.

bench/gc_race.py is the measurement: three programs written once in iyi
and once in Go, under iyi's collector, under Boehm as Crystal ships it and
under Go's. On the twenty-core Linux machine GC_DESIGN.md records, the
longest pause is Go's or under it on every program, the wall time is under
Go's on all three and under Boehm's on two, and the footprint is Go's on
churn and a budget or two over it on the rest; on darwin's three-core CI
runner the shape is the same. Every mechanism has a gate that fails by name
when it is removed, and the gates run on Linux x86-64, on aarch64 under
emulation, and on darwin arm64.

Share is built: a type's marker is decided structurally on the compiler's
AST, @[Share] is trust, and a thread's block may capture only what it
carries. The tarball went on its diet — a minimal static LLVM, 70 MB on Linux
where it was 86 and 50 MB on darwin where it was 72, with the clean-room proof
unchanged — and Windows' run-time diagnosis is retired after thirty-six clean
builds of a watch that is a gate now.

.iyimod format is unchanged at v43, and identity is the released version
as ever: a 0.8.0 artifact is rejected by a 0.9.0 build and rebuilt, never
migrated.