Capture envp from the kernel-supplied stack into a DATAW slot during
_start's prologue (before CALL main), and expose it via a `rt_envp`
TEXT getter. lib/os.getenv binds the getter as `@symbol("rt_envp")
fn rtenvp() **u8` — the getter-fn pattern works around @symbol-on-let
not being supported by the compiler yet (silent miscompile otherwise).
`os.getenv(name: str) (str | void)` matches Hare's os::getenv surface:
walks the NUL-terminated envp table, "name=" prefix-matches with an
explicit `=` boundary check so prefixes don't false-match longer
names, returns the value as a borrowed str view. Empty value (env
"FOO=") returns len=0 str, not void — void is reserved for "name
not present at all".
Cohort coverage in lib/os/ostest.ww + test/wcc/974_getenv_run.c:
set / empty / unset / prefix-no-match (4 @test fns).
