mem: Step 8 — delete context global from std.sx

The `context : Context = ---;` global in `library/modules/std.sx` had
no remaining readers — all `context.X` lookups in user code resolve
through `current_ctx_ref` (Step 5), `push Context.{...}` uses an alloca
slot (Step 6), and `allocViaContext` sources from the lowering's
current ref. `emitDefaultContextInit` (the only writer) was already
removed in Step 5.

`inferExprType` for the `context` identifier now returns the registered
`Context` type when implicit-ctx is enabled, mirroring the lowering's
identifier-handling fast path. Without this, `context.allocator` would
type as `s64` (the fallback) and the field access would fail.

11 JNI/ObjC IR snapshots regen — the `@context` LLVM global is gone
from each.

152/152 example tests pass.

tests/expected/ffi-objc-call-03-selector-sharing.ir

@@ -1,5 +1,4 @@
 
-@context = internal global { { ptr, ptr, ptr }, ptr } zeroinitializer
 @OS = internal global i64 0
 @ARCH = internal global i64 0
 @POINTER_SIZE = internal global i64 8