js-on-sx: baseline commit (278/280 unit, 148/148 slice, runner stub)

Initial commit of the lib/js/ tree and plans/ directory. A previous
session left template-string work in progress — 278/280 unit tests pass
(2 failing: tpl part-count off-by-one, escaped-backtick ident lookup).
test262-runner.py and scoreboard are placeholders (0/8 with 7 timeouts);
fixing the runner is the next queue item.

lib/js/test262-slice/README.md

@@ -0,0 +1,31 @@
+# JS-on-SX cherry-picked conformance slice
+
+A hand-picked slice inspired by test262 expression tests. Each test is one
+JS expression in a `.js` file, paired with an `.expected` file containing
+the SX-printed result that `js-eval` should produce.
+
+Run via:
+
+    bash lib/js/conformance.sh
+
+The slice intentionally avoids anything not yet implemented (statements,
+`var`/`let`, `function`, regex, template strings, prototypes, `new`,
+`this`, classes, async). Those land in later phases.
+
+## Expected value format
+
+`js-eval` returns SX values. The epoch protocol prints them thus:
+
+| JS value         | Expected file contents |
+|------------------|-----------------------|
+| `42`             | `42`                  |
+| `3.14`           | `3.14`                |
+| `true` / `false` | `true` / `false`      |
+| `"hi"`           | `"hi"`                |
+| `null`           | `nil`                 |
+| `undefined`      | `"js-undefined"`      |
+| `[1,2,3]`        | `(1 2 3)`             |
+| `{}`             | `{}`                  |
+
+The runner does a substring match — the `.expected` file can contain just
+the distinguishing part of the result.