Also fix runtime scheduler bugs:
- g_task_creating was thread-local but set on main thread and read on worker
- g_scheduler_context was global shared by all workers instead of per-thread
- Add String_IsNull and String_Offset wrappers in lib/String.bux
- Add bux_str_is_null and bux_str_offset runtime functions in rt/runtime.c
- Update String_Replace to use String_IsNull instead of String_Len(pos)==0
- Replace pointer-to-uint casts (as uint == 0, pointer arithmetic) across apps/
- Parser newline skipping fixes in src/parser.bux
bux_sb_build returned sb->buf and then bux_sb_free freed it,
causing dangling pointers in all callers of StringBuilder_Build
followed by StringBuilder_Free (e.g. JsonParser_ParseString, Fmt).
Fix: bux_sb_build now detaches the buffer (sets sb->buf = NULL)
before returning it, so bux_sb_free only frees the sb struct.
This transfers buffer ownership to the caller.
The selfhost parser did not skip tkNewLine tokens inside expression parsing,
causing multi-line expressions (e.g. if conditions spanning lines with ||)
to be truncated. When parserParseBinaryPrec hit a newline it treated it as
an invalid primary, returning a malformed literal and leaving p.pos at the
newline. parserParseBlock then failed to find '{' and its depth-counter
fallback consumed far too many tokens, causing subsequent function bodies
to be parsed into the wrong AST nodes.
Fix: add newline skipping in parserParsePrimary, parserParseUnary, and
parserParseBinaryPrec, matching the bootstrap parser's skipNewlines() calls
in parseAnd/parseOr.
Also complete the variant cap raise from 8->9 in src/sema.bux (was already
done in ast.bux, parser.bux, hir_lower.bux).
dimgigov