mem: add explicit bail diagnostics for unhandled raw-pointer interp paths

Comptime fall-through paths used to surface as bare `CannotEvalComptime`
with no hint about the actual limitation. Now each raw-pointer Value
combination that isn't yet wired sets `Interpreter.last_bail_detail`
with a one-line explanation; `printInterpBailDiag` appends it after
the op tag:

  error: post-link callback failed: CannotEvalComptime
    (op=load/load: comptime load through raw host pointer not supported
    (IR type width not threaded)) at .../bundle.sx:N:N

Sites covered: `.load` / `.store` / `.struct_gep` / `.deref` /
`.index_gep` arms for `.int`, `.byte_ptr`, `.heap_ptr` bases;
`storeAtRawPtr`'s catch-all (now exhaustively names every rejected
Value kind); foreign-arg marshalling of unsupported aggregate shapes.

Notable behaviour change: `.deref` through a raw pointer used to
silently return the pointer-as-int unchanged. That looked like a
successful deref to callers — now it errors loudly. Aggregate
passthrough (for `*string` / `*Closure` slot deref) is preserved.

The `storeAtRawPtr` `.int`/`.float` arms still assume 8-byte width —
the Store IR op doesn't carry val's TypeId. Documented inline at the
helper: real-world comptime stores hit 8-byte fields; smaller dests
would clobber. Threading val_ty into Store is left for when a
comptime path actually hits this.

153/153 still passing. The new diagnostics fire when a comptime path
goes through an unhandled shape — verified by reading the bail text
from a synthetic test (separate issue: `#run` silently drops the error
instead of surfacing the diagnostic to the user — out of scope here).

src/core.zig

@@ -182,6 +182,7 @@ pub const Compilation = struct {
         if (self.ir_emitter) |*e| interp.build_config = &e.build_config;
         ir.Interpreter.last_bail_op = null;
         ir.Interpreter.last_bail_builtin = null;
+        ir.Interpreter.last_bail_detail = null;
         const result = interp.call(id, args) catch |err| {
             if (interp.output.items.len > 0) std.debug.print("{s}", .{interp.output.items});
             return err;