Fix JET-flagged may-be-undefined locals in _parse_head/_parse_line (#46)

The QA group's JET linting (added in #40) reports four "local variable
may be undefined" errors that fail CI on main:

- `_parse_head(head::Expr)`: the `if/elseif` over `head.head` had no
  fallthrough, so `struct_name`, `type_params`, and `super` were
  undefined for any other head. Add an `else` that errors, matching the
  existing invalid-usage handling elsewhere in the file. This makes the
  function total and removes three of the JET findings.
- `_parse_line(line::Expr)`: `val` was bound only inside `if assignment`
  but its binding is visible at the later `if assignment` read site, so
  JET cannot prove it defined. Initialize `val = nothing` alongside the
  existing `annotation = nothing`; behavior is unchanged since `val` is
  only read when `assignment` is true.

Verified locally on Julia 1.12 (CI's `julia 1`): the QA group now passes
(Aqua 11/11, JET 1/1, zero findings). Core group still passes 40/40 on
Julia 1.10, including the invalid-usage assertion.

Co-authored-by: ChrisRackauckas-Claude <accounts@chrisrackauckas.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: 3 people

src/ConcreteStructs.jl

@@ -193,6 +193,8 @@ function _parse_head(head::Expr)
     elseif head.head === :(<:)
         super = head.args[2]
         struct_name, type_params = _parse_head(head.args[1])
+    else
+        error("Invalid usage of @concrete.")
     end
 
     return (struct_name, type_params, super)
@@ -210,6 +212,7 @@ _parse_line(line) = (line, nothing)
 function _parse_line(line::Expr)
     assignment = line.head === :(=)
     annotation = nothing
+    val = nothing
     if assignment
         val = line.args[2]
         line, annotation = _parse_line(line.args[1])