Iter 10: Term::Seq sequencing operator

Adds `Term::Seq { lhs, rhs }` (serde tag "seq") as a first-class
AST node for sequencing effectful expressions. Equivalent in
behaviour to `let _ = lhs in rhs`, but the dedicated node gives the
pretty-printer and diagnostics a cleaner shape and surfaces the
intent ("run for effect, then yield rhs") to future tooling.

Typecheck: lhs must be Unit; rhs's type is the result; effects
accumulate.
Codegen: lower lhs (drop SSA), lower rhs (return).
Capture / deps walkers: recurse into both sides.

Refactored examples/list_map.ail.json's print_list to use seq
instead of `let _ = ...`. Output unchanged (2/4/6).

Hash stability: existing examples without Term::Seq serialise
bit-identical; only list_map.ail.json's hashes changed (deliberate
refactor).

Tests: 51 green (was 50). New unit test
`ailang_check::tests::seq_lhs_must_be_unit` covers the type-error
path; existing list_map e2e covers the happy path.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/DESIGN.md

@@ -186,12 +186,15 @@ hashes stay bit-identical.
   "retType": Type,
   "effects": ["<id>"...],
   "body": Term }
+{ "t": "seq", "lhs": Term, "rhs": Term }
 ```
 
 In the MVP, `do` is only a direct call to a built-in effect op (no handler).
 A `lam` term constructs an anonymous function value; free variables of
 its body are captured from the enclosing scope (see Iter 8 closure
-conversion in JOURNAL).
+conversion in JOURNAL). A `seq` term evaluates `lhs` for its effects
+(its result must be Unit) and yields `rhs`'s value — equivalent to
+`let _ = lhs in rhs`.
 
 ### Type