js-on-sx: regex literal lex+parse+transpile+runtime stub

Lexer: js-regex-context? disambiguates / based on prior token;
read-regex handles [...] classes and \ escapes. Emits
{:type "regex" :value {:pattern :flags}}.

Parser: new primary branch → (js-regex pat flags).

Transpile: (js-regex-new pat flags).

Runtime: js-regex? predicate, js-regex-new builds tagged dict with
source/flags/global/ignoreCase/multiline/sticky/unicode/dotAll/
hasIndices/lastIndex. js-regex-invoke-method dispatches .test/.exec/
.toString. js-invoke-method detects regex receivers. Stub engine
uses js-string-index-of; __js_regex_platform__ + override! let a
real engine plug in later.

Runner: repeatable --filter flags (OR'd).

308/310 unit (+30 regex tests), 148/148 slice unchanged.

lib/js/test262-runner.py

@@ -527,16 +527,16 @@ class TestResult:
     elapsed_ms: int = 0
 
 
-def discover_tests(filter_prefix: str | None) -> list[Path]:
+def discover_tests(filter_prefixes: list[str] | None) -> list[Path]:
     tests: list[Path] = []
     for p in TEST_ROOT.rglob("*.js"):
         if p.name.endswith("_FIXTURE.js"):
             continue
         if "_FIXTURE" in p.parts:
             continue
-        if filter_prefix:
+        if filter_prefixes:
             rel = p.relative_to(TEST_ROOT).as_posix()
-            if not rel.startswith(filter_prefix):
+            if not any(rel.startswith(prefix) for prefix in filter_prefixes):
                 continue
         tests.append(p)
     tests.sort()
@@ -799,7 +799,8 @@ def write_markdown(scoreboard: dict, path: Path, pinned_commit: str, elapsed_s:
 def main(argv: list[str]) -> int:
     ap = argparse.ArgumentParser()
     ap.add_argument("--limit", type=int, default=0, help="max tests to run (0 = all)")
-    ap.add_argument("--filter", type=str, default=None, help="path prefix filter")
+    ap.add_argument("--filter", type=str, action="append", default=None,
+                    help="path prefix filter (repeatable; OR'd together)")
     ap.add_argument("--per-test-timeout", type=float, default=DEFAULT_PER_TEST_TIMEOUT_S)
     ap.add_argument("--restart-every", type=int, default=500,
                     help="restart server every N tests to keep memory bounded")