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>

examples/list_map.ail.json

@@ -112,14 +112,13 @@
               ]
             },
             "body": {
-              "t": "let",
-              "name": "_print_h",
-              "value": {
+              "t": "seq",
+              "lhs": {
                 "t": "do",
                 "op": "io/print_int",
                 "args": [{ "t": "var", "name": "h" }]
               },
-              "body": {
+              "rhs": {
                 "t": "app",
                 "fn": { "t": "var", "name": "print_list" },
                 "args": [{ "t": "var", "name": "t" }]