add constcast and immutable free

TODO.md

@@ -960,6 +960,11 @@
     value-producing control-flow forms with or without an error capture
   - void fallthrough and diverging `noreturn` fallbacks remain valid
 
+50. add `constcast!` and immutable deallocation (implemented)
+  - `constcast!` restores mutability only for pointers, optional pointers, and slices while
+    preserving child type, pointer kind, optionality, length, and sentinel shape
+  - `std/mem.free` accepts immutable slices and restores mutability only at the allocator boundary
+
 ## A word on unchecked casts
 
 For casts that bypass safety checks, Honey provides builtin functions:
@@ -968,7 +973,8 @@ For casts that bypass safety checks, Honey provides builtin functions:
 | -- | -- | -- |
 | `truncate(x, T)` | Keep low bits, discard rest | Never |
 | `bitcast(x, T)` | Reinterpret bits, no cast | Sizes don't match (compile error) |
-| `ptrcast!(p, T)` | Change pointer type | Gaining mutability (compile error) |
+| `ptrcast!(T, p)` | Change a pointer's child type while preserving its shape | Invalid child or non-pointer operand (compile error) |
+| `constcast!(p)` | Restore pointer or slice mutability | Non-pointer/slice operand (compile error) |
 
 ```honey
 # truncation
@@ -981,12 +987,10 @@ m := bitcast(n, u32)  # m == 0xFFFFFFFF (same bits)
 f: f32 = 3.14
 bits := bitcast(f, u32)   # IEEE 754 representation
 
-# pointer casts (element type, many ↔ single, pointer ↔ usize)
-buf: *u8 = get_buffer()
-ints := ptrcast!(buf, *u32)    # element type change
-single := ptrcast!(buf, @u8)   # many → single (restricting)
-addr := ptrcast!(buf, usize)   # pointer to integer
-ptr := ptrcast!(addr, @u8)     # integer to pointer
+# pointer casts
+buf *u8 = get_buffer()
+ints *u32 = ptrcast!(u32, buf)       # element type change, same pointer shape
+writable *mut u8 = constcast!(buf)   # explicit unsafe mutability restoration
 ```
 
 ## A word on multi-unwrap