lib: retire os.assert/abort shims — assert/abort are builtins (#58 respell)

The flat checker scope makes ANY decl named assert/abort anywhere in
the combined unit disable the builtin unit-wide (the #45 shadow shape:
scope_lookup_prefer's cross-module fallback finds it). lib carried
three colliding @symbol("rt_abort") shims (os, time, strconv/stof)
plus the os.assert wrapper, so a bare assert(cond) in ANY program
importing os mis-bound os.assert and failed arity — a hard blocker for
regex fold-5 (regex.ha:660/670 bring builtin-assert mass). Ruled
respell-now per the recurrence test (#45 -> #58).

Delete the shims and the os.assert wrapper; every bare abort(msg)
caller (regex, strings, utf8, hash, getopt, encoding/*, time, stof)
now lands on the builtin, and the ~40 os.assert(c, m) sites respell to
the builtin assert(c, m) — restoring the exact Hare spelling the lib
ports diverged from (e.g. ref/hare/bytes/tokenize.ha:23). os.assert
had no Hare counterpart (Hare's assert is a language builtin); rule-9
wrapper removed. temp/dirs/bufio already use the non-colliding rtabort
spelling and keep it.

Now-dead 'import os;' lines kept (pre-existing precedent:
lib/strconv/strconv.ww carries one); a tree-wide dead-import sweep is
a separate concern. regex.ww's if+abort workarounds citing #58 stay
for the fold-5 owner to fold back into assert.

combined.ww regenerated for all five selfhost tools + the smoke
fixture via make.

lib/strings/strings.ww

@@ -223,7 +223,7 @@ fn utf8bytelenbounded(it: *iterator, end: i32) i32 = {
 // defaulting end=END is omitted: ww has no default-parameter syntax
 // (filed as #37). Byte-indexed counterpart: [[bytesub]].
 export fn sub(s: str, start: i32, end: i32) str = {
-	os.assert(start <= end, "strings.sub: start is higher than end");
+	assert(start <= end, "strings.sub: start is higher than end");
 	let it: iterator = iter(s);
 	let starti: i32 = utf8bytelenbounded(&it, start);
 	let endi: i32 = utf8bytelenbounded(&it, end - start);
@@ -239,8 +239,8 @@ export fn sub(s: str, start: i32, end: i32) str = {
 // codepoint); the equivalent Hare predicate is `s[i] & 0xc0 == 0x80`
 // at ref/hare/strings/sub.ha:72-73.
 export fn bytesub(s: str, start: i32, end: i32) (str | utf8.invalid) = {
-	os.assert(start <= end, "strings.bytesub: start is higher than end");
-	os.assert(end <= s.len, "strings.bytesub: end exceeds string length");
+	assert(start <= end, "strings.bytesub: start is higher than end");
+	assert(end <= s.len, "strings.bytesub: end exceeds string length");
 	if (start < s.len && (s[start] & 0xC0u8) == 0x80u8) {
 		let e: utf8.invalid; return e;
 	};
@@ -668,7 +668,7 @@ export fn tokenize(s: str, delim: str) tokenizer = {
 	let d: []u8 = toutf8(delim);
 	let i: i32 = 0;
 	for (i < d.len) {
-		os.assert((d[i] & 0x80u8) == 0u8,
+		assert((d[i] & 0x80u8) == 0u8,
 			"strings.tokenize cannot tokenize on non-ASCII delimiters");
 		i += 1;
 	};
@@ -682,7 +682,7 @@ export fn rtokenize(s: str, delim: str) tokenizer = {
 	let d: []u8 = toutf8(delim);
 	let i: i32 = 0;
 	for (i < d.len) {
-		os.assert((d[i] & 0x80u8) == 0u8,
+		assert((d[i] & 0x80u8) == 0u8,
 			"strings.rtokenize cannot tokenize on non-ASCII delimiters");
 		i += 1;
 	};