Fix L2: replace unreachable! with Err for untrusted-schema safety

The immediate downstream consumer (alkcall) accepts schemas from
arbitrary internet peers in its hub/spoke topology. A panic is the
wrong failure mode for a malicious or unsupported schema - an Err
the caller can handle is correct.

Three sites converted:
- offset_map.rs: _ => Err(Offset { unsupported AlkType kind for
  aligned offset computation })
- layout_builder.rs: _ => Err(Offset { unsupported AlkType kind
  for packed layout computation })
- materialize.rs: _ => Err(Schema { internal: union discriminator
  type N is not a supported byte discriminator })

The materialize.rs site uses Schema (not Access) because a wrong
disc_type is a schema-authoring bug (parse_discriminator should have
caught it), not a buffer-access error. The sibling sites in
sequential_reader.rs and tunion.rs already returned Err(Schema) -
only materialize.rs was the holdout.

Note: the k if k.is_fixed_size() guard in offset_map and
layout_builder means the compiler cannot enforce exhaustiveness at
compile time. Converting _ from unreachable! to Err is the runtime
mitigation. A future refactor could list all fixed-size kinds
explicitly to restore compile-time checking. Deferred to a separate
cleanup pass.

Verified: no unreachable! remains in production code (the one
remaining hit at offset_map.rs:688 is inside a #[test] fn).

cargo test --release: 396 tests pass, 0 failures
cargo clippy --all-targets -- -D warnings: clean
cargo build --target wasm32-unknown-unknown --release: clean (prior)

src/offset_map.rs

@@ -274,7 +274,10 @@ impl<'a> ComputeCtx<'a> {
             k if k.is_fixed_size() => {
                 self.compute_fixed_field(k, field_schema, field_path, struct_default_align)
             }
-            _ => unreachable!("all AlkTypeKind variants are covered above"),
+            other => Err(AlkTypeError::Offset {
+                field_path: field_path.to_string(),
+                reason: format!("unsupported AlkType kind {other} for aligned offset computation"),
+            }),
         }
     }