stdlib: relocate modules under library/

- examples/modules/ -> library/modules/ (top-level, no more
  symlink hacks in consumer projects)
- compiler discovers stdlib via _NSGetExecutablePath / readlink
  /proc/self/exe; searches dev layout (../../library), install
  layout (../library), and alongside-binary fallback
- SX_STDLIB_PATH env var overrides for tests / dev convenience
- SX_DEBUG_STDLIB env var dumps the discovery results
- build.zig installs library/ alongside the binary
- Compilation gains stdlib_paths field threaded through resolveImports
- 50 tests pass; consumer projects can now build from any cwd

library/modules/math/math.sx

@@ -0,0 +1,37 @@
+PI :f32: 3.14159265;
+TAU :f32: 6.28318530;
+DEG2RAD :f32: 0.01745329;
+RAD2DEG :f32: 57.2957795;
+
+sqrt :: (x: $T) -> T #builtin;
+sin :: (x: $T) -> T #builtin;
+cos :: (x: $T) -> T #builtin;
+floor :: (x: $T) -> T #builtin;
+
+min :: (a: $T, b: T) -> T {
+    if a < b then a else b;
+}
+
+max :: (a: $T, b: T) -> T {
+    if a > b then a else b;
+}
+
+clamp :: (val: $T, lo: T, hi: T) -> T {
+    if val < lo then lo
+    else if val > hi then hi
+    else val;
+}
+
+abs :: (x: $T) -> T {
+    if x < 0 then 0 - x else x;
+}
+
+lerp :: (a: f32, b: f32, t: f32) -> f32 {
+    a + (b - a) * t;
+}
+
+sign :: (x: $T) -> T {
+    if x > 0 then 1
+    else if x < 0 then 0 - 1
+    else 0;
+}