rt+lib/os+test: capture envp; add os.getenv

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).

Makefile

@@ -234,7 +234,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_w6c_ww $(BIN)/test_ww_ww $(BIN)/test_self_rebuild \
 	  $(BIN)/test_dyn_ww $(BIN)/test_selfcheck $(BIN)/test_at_test_ww \
 	  $(BIN)/test_fmt_run $(BIN)/test_log_run $(BIN)/test_fnmatch_run \
-	  $(BIN)/test_shlex_run \
+	  $(BIN)/test_shlex_run $(BIN)/test_getenv_run \
 	  $(BIN)/test_memio_run $(BIN)/test_temp_run $(BIN)/test_getopt_run \
 	  $(BIN)/test_base32_run $(BIN)/test_base64_run \
 	  $(BIN)/test_adler32_run $(BIN)/test_crc16_run \
@@ -448,6 +448,10 @@ $(BIN)/test_shlex_run: test/wcc/973_shlex_run.c $(BIN)/ww $(BIN)/w6c \
 		$(BIN)/w6a $(BIN)/w6l $(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_getenv_run: test/wcc/974_getenv_run.c $(BIN)/ww $(BIN)/w6c \
+		$(BIN)/w6a $(BIN)/w6l $(LIB)/libwwrt.a | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 $(BIN)/test_memio_run: test/wcc/980_memio_run.c $(BIN)/ww $(BIN)/w6c \
 		$(BIN)/w6a $(BIN)/w6l $(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<