fibers B1.1: per-fiber context root is library-only (no compiler change)

A fiber needs its own root Context (the spawner's snapshot), not the
ambient one. Probed whether that needs compiler support: it does not.
context is an implicit slot-0 *Context param (call-carried, rides the
callee's own stack) and push Context allocates on the caller frame —
never TLS, never re-read from the __sx_default_context global mid-stack.
So the spawn convention is pure library sx:

  snap := context;            // snapshot the spawner's context
  f := Fiber.{ root = snap }; // store it
  push f.root { entry(args) } // trampoline installs it as the fiber root

examples/1804-concurrency-context-snapshot.sx locks it: a trampoline
running under ambient ctx 99 installs a stored snapshot (42); the body
reads 42, and the push scope restores 99 on exit. No fiber runtime yet
(B1.3) — this proves the plumbing it builds on.

The design doc's "lower context as swappable indirection, never raw
TLS" guarded a non-problem — context was already param-carried.

Suite green (726/0).

examples/expected/1804-concurrency-context-snapshot.exit

@@ -0,0 +1 @@
+0

examples/expected/1804-concurrency-context-snapshot.stderr

@@ -0,0 +1 @@
+

examples/expected/1804-concurrency-context-snapshot.stdout

@@ -0,0 +1,2 @@
+fiber root: 42
+ambient after: 99