feat: infer complete ptrcast target types

TODO.md

@@ -176,8 +176,8 @@
   - `Name :: opaque` is the incomplete nominal record spelling; bodyless `c_struct` is invalid
   - `anyopaque` is the erased object type used behind pointers for C `void*` and allocator contexts
   - C `void` function results remain `void`; C `void*` / `const void*` import and render as `?*mut anyopaque` / `?*anyopaque`
-  - `ptrcast!(T, ptr)` preserves pointer shape and only changes the child type in v1
-  - future direction: generalize toward Zig-style arbitrary pointer-result casts once casts have a broader result-type story
+  - `ptrcast!(ptr)` infers its complete pointer result from context; `ptrcast!(P, ptr)` accepts an explicit complete pointer type
+  - both forms change the child type, then permit only ordinary implicit pointer conversions to the requested target; broader pointer-shape changes remain deferred
 
 12. `undefined` as inspired by zig (implemented):
   - allow mutable local and global declarations with `undefined`
@@ -973,7 +973,7 @@ 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!(T, p)` | Change a pointer's child type while preserving its shape | Invalid child or non-pointer operand (compile error) |
+| `ptrcast!(p)` / `ptrcast!(P, p)` | Infer or explicitly supply a complete pointer target, changing the child before ordinary pointer coercion | Missing target context, invalid target, incompatible shape, or non-pointer operand (compile error) |
 | `constcast!(p)` | Restore pointer or slice mutability | Non-pointer/slice operand (compile error) |
 
 ```honey
@@ -989,8 +989,10 @@ bits := bitcast(f, u32)   # IEEE 754 representation
 
 # 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
+ints *u32 := ptrcast!(buf)                          # target inferred from the declaration
+raw ?*mut anyopaque := get_raw_buffer()
+bytes := ptrcast!(?*mut u8, raw)                   # complete target supplied explicitly
+writable *mut u8 := constcast!(buf)                 # explicit unsafe mutability restoration
 ```
 
 ## A word on multi-unwrap