* make the green backend the default on linux

green is faster on every shape this repo measures and the last blocker
(dns holding a worker) landed in #598, so a spawned task now runs as a
coroutine on the worker pool unless you say otherwise.

the default stays os threads everywhere else. the reactor is epoll and
eventfd, so macos and the bsds compile netpoll_fallback, which has no
reactor at all: a green task waiting on a socket there would hold its
worker for the whole wait. green is still reachable with PITH_GREEN=1 on
those platforms, just not the default.

PITH_GREEN now reads in both directions. 1/on/true force green, 0/off/false
force os threads, and anything else takes the platform default. the off
side is new and is the escape hatch for anyone the flip hurts, so it is
trimmed and matched case-insensitively. an unrecognized value falls to the
platform default rather than to os threads, so the only values that move
the backend are the ones spelled out in the match and a typo behaves
exactly like an unset variable.

parsing moved into a pure backend_from_env so it can be tested without
fighting the OnceLock that backend() caches into.

* pin both backends explicitly in the differential tests

with green as the linux default, a comparison whose reference side just
says `pith run` is green against green and passes for free. every green-*
target now runs its os-thread side under PITH_GREEN=0.

the same reasoning applies to coverage. `make test` on a linux box now
exercises only green, so the PITH_GREEN=0 path across the corpus had no
gate at all. verify-osthread-corpus mirrors verify-green-corpus for the
opt-out and ci runs both. memcheck grew a small second list for the cases
whose subject is the os-thread task machinery itself, so the flip does not
quietly stop testing the code they were written for.

* describe green as the default rather than the experiment

docs/concurrency.md drops the "off by default and still experimental"
framing. the backend-choice section now says green on linux, os threads
elsewhere, and PITH_GREEN=0 as the opt-out, and it keeps the caveats that
came with the flip rather than burying them: file i/o has no yield point
so a file read holds its worker and everything pinned to it; preemption is
a build-time opt-in where the kernel gave os threads preemption for free;
and task placement is first-resume luck.

docs/limitations.md had a list of what it would take to make green the
default. most of that is history now, so it reads as the caveats of the
current default instead.

the performance scoreboard labelled its concurrency rows "(green)"; the
os-thread rows are the ones that need a label now, so they carry
PITH_GREEN=0. the same for the bench README table and the examples and
module headers that showed PITH_GREEN=1 as the way to opt in.

* accept no/yes for the backend switch, and fix two stale strings

three small corrections on top of the flip.

`PITH_GREEN=no` fell to the platform default, which on linux is green — so
someone reaching for the escape hatch and writing the most natural word for
"off" got exactly what they were trying to turn off. no/n now mean os threads
and yes/y mean green, which keeps the rule (only spelled-out values move the
backend) while making the direction that costs something hard to miss.

the green corpus target claimed it "matches os-thread output". it compares
against a fixed expected file, which is the stronger check and the reason it
catches a drift both backends share. now that there is a sibling target that
really does run os threads, the old wording is worth correcting.

and the corpus is 261 cases, not 260, since the dns case joined it.