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/strconv/decimal.ww

@@ -22,7 +22,7 @@
 //     (when no continue exists in the loop).
 //   - Hare `fn foo() T = if (cond) {...} else expr;` expression body
 //     → ww requires a `{}` block body throughout.
-//   - Hare bare `assert(cond)` builtin → `os.assert(cond, msg)`;
+//   - Hare bare `assert(cond)` builtin → `assert(cond, msg)`;
 //     wwstage cgen has no `assert` intercept (deferred fold).
 //   - In-file instances of the above hoist pattern: `i_sz` (line 93)
 //     hoists a per-iteration size cast out of a for-loop comparison
@@ -110,7 +110,7 @@ fn leftshift(d: *decimal, k: u32) void = {
 	let SZ_BOUND: size = (len(d.digits): size);
 	let kU64: u64 = (k: u64);
 	let MAXSHIFT_U32: u32 = (maxshift: u32);
-	os.assert(k <= MAXSHIFT_U32, "strconv.leftshift: k > maxshift");
+	assert(k <= MAXSHIFT_U32, "strconv.leftshift: k > maxshift");
 	if (d.nd == (0u64: size)) { return; };
 	let nn: u32 = leftshift_newdigits(d, k);
 	let r: int = (d.nd: int) - 1;