fix(0138): diagnose @scalar-const address-of (no storage)

A scalar `::` constant folds to its value and has no storage. The
unary `.address_of` lowering (src/ir/lower/expr.zig) skipped the
alloca path (is_alloca == false) and resolveGlobalRef (scalar consts
get no storage global), falling through to the generic addr_of arm,
which reinterpreted the folded value as a pointer:
`inttoptr (i64 <value> to ptr)`. That wild pointer segfaulted on
deref and emitted invalid stores for inline-asm `-> @const`.

Diagnose instead, in the address_of(identifier) path: a non-alloca,
non-ref-capture, non-pack-elem scope binding (local scalar const) and
a module_const_map name not backed by storage (module scalar const)
both report "cannot take the address of constant '<name>' — a scalar
'::' constant has no storage …" and return a placeholder Ref. Chose
diagnose over materializing read-only storage (consistent with the
fold-only scalar model). Array/struct consts keep real storage and
stay addressable (@K/@LIT unchanged).

Also gives the ASM stream's planned output-to-const rejection for
free — asm `-> @const` lowers through the same path. Regression:
examples/1177-diagnostics-addr-of-const-rejected.sx. Resolves 0138.

examples/1177-diagnostics-addr-of-const-rejected.sx

@@ -0,0 +1,18 @@
+// Taking the address of a scalar `::` constant is a compile error: a scalar
+// constant folds to its value and has NO storage (only array/struct constants
+// are immutable globals with a real address — see 0177). Covers a module-scope
+// const, a local const, and an inline-asm `-> @const` write-through (the path
+// that surfaced the bug). Before the fix, `@N` lowered to `inttoptr (i64 40 to
+// ptr)` — a wild pointer that segfaulted on deref and emitted invalid stores
+// for asm `-> @const`. Regression (issue 0138).
+
+takes :: (p: *i64) {}
+
+N :: 40;
+
+main :: () {
+    takes(@N);          // module scalar const — no storage
+    x :: 7;
+    takes(@x);          // local scalar const — no storage
+    asm volatile { "mov %[c], #99", [c] "=r" -> @N };   // write-through to a const
+}

examples/expected/1177-diagnostics-addr-of-const-rejected.exit

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

examples/expected/1177-diagnostics-addr-of-const-rejected.stderr

@@ -0,0 +1,17 @@
+error: cannot take the address of constant 'N' — a scalar '::' constant has no storage (use a '=' variable or a local copy for mutable data)
+  --> examples/1177-diagnostics-addr-of-const-rejected.sx:14:11
+   |
+14 |     takes(@N);          // module scalar const — no storage
+   |           ^^
+
+error: cannot take the address of constant 'x' — a scalar '::' constant has no storage (use a '=' variable or a local copy for mutable data)
+  --> examples/1177-diagnostics-addr-of-const-rejected.sx:16:11
+   |
+16 |     takes(@x);          // local scalar const — no storage
+   |           ^^
+
+error: cannot take the address of constant 'N' — a scalar '::' constant has no storage (use a '=' variable or a local copy for mutable data)
+  --> examples/1177-diagnostics-addr-of-const-rejected.sx:17:49
+   |
+17 |     asm volatile { "mov %[c], #99", [c] "=r" -> @N };   // write-through to a const
+   |                                                 ^^

examples/expected/1177-diagnostics-addr-of-const-rejected.stdout

@@ -0,0 +1 @@
+