Port head-only of cstage cmd/wcc/check.c:1437-1451 to exprtype's
new N_TUPLE arm. The arm walks e.list, types each element via
recursive exprtype, and assembles an N_TTUPLE whose .list chains
N_TPARAM wrappers (one per element) so shared element-type ASTs
(sym.decl.lhs, struct field's .lhs, another tuple's element) keep
their own .next untouched.
Foundation: A.6.2.0b-pre (805c841) introduced N_TPARAM as the
chain wrapper for N_TTUPLE.list, mirroring cstage's Tparam at the
AST layer (cstage keeps it at the Type layer; wwstage has no
separate type layer). This commit is the first checker-side
synthesizer to use it.
One documented divergence: empty list returns nil under the
lenient policy (cstage would synthesize empty TY_TUPLE; ww grammar
requires >= 2 elements per parse/expr.ww:117, so empty is
unreachable either way). Same shape as scruttype L656 / A.6.1.5b
N_DOT lenient-on-miss precedent.
α scope per Drew (no hint plumbing, no field-level walks, no
unify check). Hint param stays unused.
A.6.2 step 2 of 8 (γ order). N_ALLOC next (most likely a
parser-vestigial kind — A.6.2.0c worker audits first).
`make test-unit` green; full `make test` deferred to the batched
gate after the A.6.2.1 assertion commit (option B per user).
Hojun-Cho
1131e7b68d