Fix JET-flagged may-be-undefined locals failing QA CI

[ChrisRackauckas-Claude]

What

Fixes the failing QA CI on main. The QA group's JET linting (JET.test_package, added in #40) reports four "local variable may be undefined" errors and fails the tests / QA (julia 1) lane:

_parse_head(head::Expr) @ src/ConcreteStructs.jl:198
  local variable `struct_name` may be undefined
  local variable `type_params` may be undefined
  local variable `super` may be undefined
_parse_line(line::Expr) @ src/ConcreteStructs.jl:228
  local variable `val` may be undefined

These are genuine latent issues in the source, surfaced by the JET lint check that was newly added to the QA group; they are not fixed by skipping/loosening anything.

Fix (3 lines, behavior-preserving)

• _parse_head(head::Expr) — the if head.head === :curly / elseif === :(<:) chain had no fallthrough, so for any other head the three returned locals were undefined. Added an else that error("Invalid usage of @concrete."), matching the existing invalid-usage handling in _find_struct_def/_apply_original_expr. This makes the function total and clears three findings.
• _parse_line(line::Expr) — val was assigned only inside if assignment, but its binding is read at the later if assignment ? ... : ... site, which JET cannot prove reachable-only-when-defined. Initialized val = nothing next to the existing annotation = nothing. No behavior change: val is only read when assignment is true.

Local verification

• Julia 1.12 (CI's julia 1), QA sub-env: Aqua 11/11 and JET 1/1 pass, zero JET findings.
• Julia 1.10 (compat floor), Core group: 40/40 pass, including the invalid-usage assertion.
• Runic --check on src/ConcreteStructs.jl: clean (exit 0).

Please ignore until reviewed by @ChrisRackauckas.