docs: alias carry semantics + stdlib layout (step 4)

- specs §9: Namespace Alias Carry section (one level, own-wins, ambiguity,
  no chaining — 0114 noted for the still-ungated bare-call path), the
  three-tier import resolution (file dir -> cwd -> stdlib search path /
  SX_STDLIB_PATH), a Standard Library Layout section, real-layout examples
  replacing the stale modules/std/std.sx ones.
- readme: carry-rule teaser with the std namespace-tail example (verified
  to compile and run as written).
- CLAUDE.md: file-roles rows for std.sx/std//ffi//math//build.sx,
  tests/fixtures, and the PLAN-STDLIB tracker.

readme.md

@@ -441,6 +441,26 @@ struct / pack fn / protocol body is instantiated in the module that defines it,
 e.g. `List(T).append`'s `alloc: Allocator` is visible there regardless of the call
 site.)
 
+**Namespace aliases carry one level.** A namespaced import is an ordinary
+declaration, and flat-importing the module that declares it makes the alias
+usable in the importer — there is no `pub` keyword. The stdlib prelude uses
+exactly this: `#import "modules/std.sx"` gives every bare prelude name
+(`print`, `List`, `Context`, …) plus the carried namespaces of std's tail:
+
+```sx
+#import "modules/std.sx";
+
+main :: () {
+    gpa := mem.GPA.init();          // mem :: #import — carried from std.sx
+    log.warn("count = {}", 3);
+    s := xml.escape("<a & b>");
+}
+```
+
+Carried aliases follow declaration rules: an own declaration shadows a carried
+alias, two flat imports carrying the same alias make its use ambiguous, and
+carry does not chain through a second flat hop.
+
 ### Implicit Context
 
 Every program gets an implicit `context` with a default allocator: