cstage+selfhost+test: refuse same-block let / param redecl (#32)

cmd/wcc/check.c silently accepted `let a; let a;` in the same block
and similar redecls. Pre-#27 the localoff dedup masked it; post-#27
last-write-wins via head-first localfind. Surfaced by worker-27
during the #27 review.

Cstage: 5 guard sites (check_scope_define-NULL → err) covering
N_LET block-bind, N_MLET tuple binders (incl. same-tuple
`let (a,a)`), N_FORRANGE tuple binders, top-level let, fn param.
Voice: "<kind> '<name>' redeclared in same scope" for inner;
"duplicate let %s" for top-let, matching the existing
"duplicate <kind>" idiom at 1812/1851/1872.

Wwstage: TODO(#11) comments at the 4 mirror sites (installdecl,
N_FORRANGE, N_LET, installparams). Full enforcement waits on the
checkfile pass per rob.

**Unmasked by #32 (worth flagging):** selfhost/cmd/wcc/cgenexpr.ww
cgcall had `let callee: *node = n.lhs;` twice at fn-body scope
(copy-paste, identical value). Pre-fix silent-redecl absorbed it;
post-fix the new guard rejects. Removed the second decl — outer
`callee` stays visible across the intermediate block.

Test 712 (redecl): 10 rows (6 neg + 4 pos), cstage-only per rob.
Negative rows cover all 5 guard sites + same-tuple-dup. Positive
rows pin the legal counter-shapes (cross-block, name-only bucket,
forrange body, mcase-per-arm).

Test 300 row 34 ("shadowing in inner scope; same scope flagged")
was incorrectly asserting the bug; flipped to expect "redeclared"
and added a sibling row pinning cross-block shadow stays ok. Test
709's `same_block_redecl_pin` canary (explicitly documented as
flipping under #32) removed; pointer to 712 left in its place.

test/wcc/300_check.c

@@ -96,7 +96,9 @@ static const struct row rows[] = {
 	{ "fn f() void = { -true; };", "non-numeric" },
 	{ "fn f() void = { *5; };", "deref non-pointer" },
 	{ "fn f() void = { let x: i32 = 1; let x: i32 = 2; };",
-	  "ok" },	/* shadowing in inner scope; same scope flagged */
+	  "redeclared" },	/* same-scope let-redecl rejected (#32). */
+	{ "fn f() void = { let x: i32 = 1; { let x: i32 = 2; }; };",
+	  "ok" },	/* nested-block shadow stays legal. */
 	{ "fn f() void = { break; };", "break outside loop" },
 	{ "fn f() void = { continue; };", "continue outside loop" },
 	{ "fn f(x: i32) void = { x[0]; };", "indexing non-indexable" },