mem: implicit-context foundation + many compiler fixes

The session-long set of changes that lay the groundwork for the
Jai-literal implicit-Context-parameter refactor. Lots of accumulated
work; the new arrival is the implicit-ctx foundation (steps 1+2 of
the plan in current/CHECKPOINT-MEM.md):

  Step 1 — `CAllocator :: struct {}` stateless allocator in
    library/modules/allocators.sx, delegating directly to
    libc_malloc/libc_free. `ConstantValue` in src/ir/inst.zig gains a
    `func_ref: FuncId` leaf so nested aggregates can carry function
    pointers (the inline Allocator value's fn-ptr fields). Switch
    sites updated in emit_llvm.zig, print.zig, interp.zig.

  Step 2 — `emitDefaultContextGlobal` in src/ir/lower.zig synthesises
    a static `__sx_default_context` global with a nested-aggregate
    init_val pointing at the CAllocator → Allocator thunks. The
    second-pass `initVtableGlobals` in emit_llvm.zig is generalised
    to handle `.aggregate` init_vals (re-emits after func_map is
    populated so func_ref leaves resolve to real symbols).

Also folded in from earlier work this session:

  - Phase 1.1: `xx value` heap-copy in `buildProtocolValue` routes
    through `context.allocator` via the new `allocViaContext` helper.
  - interp.zig: `marshalForeignArg` double-offset bug fixed —
    `heapSlice` already adds `hp.offset` to the slice ptr, so the
    extra `+ hp.offset` was scribbling memcpy/memset into adjacent
    heap state, corrupting `heap.items[0]`. Symptom: `build_format`
    at comptime produced zero bytes, all `print` calls failed.
  - Lazy lowering: `lazyLowerFunction` now declares foreign-body
    functions as extern stubs in the local (comptime) module so
    cross-module foreign calls resolve.
  - Allocator API: all stdlib allocators on one-line `init() -> *T`
    (CAllocator/GPA: libc-backed; Arena/TrackingAllocator: parent-
    backed; BufAlloc: embeds state at head of user buffer).
  - issues 0038 (transitive #import), 0039 (chess + stdlib migration
    fallout), 0040 (generic struct method dot-dispatch), 0041
    (pointer types as type-arg), 0042 (alias name resolution) — all
    fixed; regression tests in examples/.
  - Diagnostic: `emitError` now embeds the lowering's
    `current_source_file` and enclosing function in the literal
    message; SX_TRACE_UNRESOLVED=1 dumps a Zig stack trace at the
    emit site so misattributed spans can't hide where the failure
    is.
  - tools/verify-step.sh (all-platforms gate) and tools/scratch.sh
    (interp/codegen parity tester) added.

Test suite: 152 example tests pass; chess builds + screenshots on
macOS / iOS sim / Android.

examples/issue-0042.sx

@@ -0,0 +1,22 @@
+// issue-0042 — const-decl type alias must resolve through
+// `type_alias_map` when used as a `$T: Type` argument to size_of /
+// align_of, not silently fall back to .s64 (8 bytes).
+// Also covers identifier-RHS aliases (chains + struct aliases),
+// not just *T / [N]T / ?T forms.
+#import "modules/std.sx";
+
+MyInt   :: s32;
+MyChain :: MyInt;
+Wide    :: struct { a: s64; b: s64; }
+WideAlias :: Wide;
+
+main :: () -> s32 {
+    print("direct s32:        {}\n", size_of(s32));
+    print("alias s32:         {}\n", size_of(MyInt));
+    print("chain s32:         {}\n", size_of(MyChain));
+    print("align alias:       {}\n", align_of(MyInt));
+    print("align chain:       {}\n", align_of(MyChain));
+    print("size struct-alias: {}\n", size_of(WideAlias));
+    print("align struct-alias:{}\n", align_of(WideAlias));
+    0;
+}