fix(gen, gen-c): lower ExprCast in Zig and C emitters (#1337)

* fix(gen,gen-c): lower ExprCast in Zig and C emitters

Companion to #1320 (which added the Rust arm). NodeKind::ExprCast now
has a lowering arm in all three text emitters; only gen-verilog previously
had one.

- gen (Zig), gen_expr default was '_ => {}' (silent drop): now emits
  '@as(<target>, @intcast(<operand>))'. @as establishes the explicit target
  type, @intcast does the safe integer conversion. Works in any expression
  position and for both narrowing (u32->u8) and widening (u8->u32) — better
  than the @as-only minimal pass suggested in #1333, which cannot narrow.
- gen-c, gen_c_expr default emitted '/* unsupported: ExprCast */': now emits
  '((<target_c_type>)(<operand>))' using the existing Self::type_to_c helper
  (u8->uint8_t, u32->uint32_t).

Closes #1333.

Verified:
- t27c self-tests: 20 passed, 0 failed.
- 't27c gen' on tri-net specs/wire.t27: be_byte -> '@as(u8, @intcast((w >> 24) & 255))',
  u32_be -> all 'as u32' casts now '@as(u32, @intcast(...))'. Previously silent drop.
- 't27c gen-c' on wire.t27: be_byte -> '((uint8_t)(((w >> 24) & 255)))'.
  Previously '/* unsupported: ExprCast */'.
- Minimal repro 'fn hi_byte(w: u32) -> u8 { return ((w >> 24) & 255) as u8; }'
  -> gen: 'return @as(u8, @intcast((w >> 24) & 255));',
     gen-c: 'return ((uint8_t)(((w >> 24) & 255)));'.
- comprehensive_suite.t27: 0 'unsupported: ExprCast' in both gen and gen-c output.

With #1320 (Rust) + this (Zig, C), ExprCast is fully lowered across all four
backends (gen-verilog already had it). Unblocks multi-target T27-first flips
in downstream consumers (tri-net gen/zig + gen/c drift-guard).

* docs(now): add exprcast-zig-c-emitters section (Closes #1333)

Wave Loop 411 close-out requires docs/NOW.md to reflect every landing PR.
Adds a section for this PR documenting:
- WHERE: gen/gen-c ExprCast arms in bootstrap/src/compiler.rs
- Zig form: @as(<T>, @intcast(<operand>)) -- narrows and widens
- C form: ((<uintN_t>)(<operand>)) via Self::type_to_c
- Coverage: all 4 backends now lower ExprCast (Rust from #1320, Verilog
  already, Zig+C in this PR)
- Downstream unblock: tri-net multi-target drift-guard (Rust+Zig+C from
  specs/wire.t27) can now proceed
- Regression: t27c self-tests 20/0; comprehensive_suite.t27 shows 0
  'unsupported: ExprCast' in Zig+C output

Anchor: phi^2 + phi^-2 = 3

---------

Co-authored-by: SSD DDD <ssdm4@MacBook-Pro.local>
Co-authored-by: Perplexity Computer <agent@perplexity.ai>

Author: 3 people

bootstrap/src/compiler.rs

@@ -3624,6 +3624,20 @@ impl Codegen {
                 }
                 self.write(" }");
             }
+            NodeKind::ExprCast => {
+                if !node.children.is_empty() {
+                    let target = node
+                        .extra_type
+                        .split('[')
+                        .next()
+                        .unwrap_or("")
+                        .trim()
+                        .to_string();
+                    self.write(&format!("@as({}, @intCast(", target));
+                    self.gen_expr(&node.children[0]);
+                    self.write("))");
+                }
+            }
             _ => {}
         }
     }
@@ -5970,6 +5984,19 @@ impl CCodegen {
                     self.gen_c_expr(&node.children[0]);
                 }
             }
+            NodeKind::ExprCast => {
+                if !node.children.is_empty() {
+                    let target = node
+                        .extra_type
+                        .split('[')
+                        .next()
+                        .unwrap_or("")
+                        .trim();
+                    self.write(&format!("(({})(", Self::type_to_c(target)));
+                    self.gen_c_expr(&node.children[0]);
+                    self.write("))");
+                }
+            }
             _ => {
                 self.write(&format!("/* unsupported: {:?} */", node.kind));
             }

docs/NOW.md

@@ -39,6 +39,13 @@
 
 # NOW — Wave Loop 411 close-out / Wave Loop 412 setup
 
+## exprcast-zig-c-emitters -- lower ExprCast in gen (Zig) and gen-c (C) emitters (Closes #1333)
+
+- **WHERE** (compiler emitters): `bootstrap/src/compiler.rs`. gen (Zig) `gen_expr` default arm `_ => {}` (silent drop for ExprCast) replaced with an `@as(<T>, @intCast(<operand>))` form -- stronger than the issue's `@as`-only suggestion because `@intCast` correctly narrows as well as widens. gen-c (C) `gen_c_expr` default `/* unsupported: ExprCast */` replaced with `((<uintN_t>)(<operand>))` via `Self::type_to_c`. Rust arm (compiler.rs:8172 from #1320) unchanged; Verilog arm already lowered ExprCast. All four backends now lower ExprCast.
+- **Why**: with #1320 Rust-only, `t27c gen` and `t27c gen-c` silently produced empty output or `/* unsupported */` for any T27 program using `as`-casts. Downstream `gHashTag/tri-net` needed this for multi-target drift-guard (Rust + Zig + C from a single `specs/wire.t27`); without ExprCast in Zig/C the guard would trip on the first cast (`be_byte`, `u32_be`) and no Zig/C-target port could ship. Verified on real wire.t27: `be_byte` emits `@as(u8, @intCast((w >> 24) & 255))` in Zig and `((uint8_t)(((w >> 24) & 255)))` in C; `u32_be` emits the four `@as(u32, @intCast(...))` widen-then-shift terms. Regression: `t27c` self-tests 20/0; `comprehensive_suite.t27` contains 0 occurrences of `unsupported: ExprCast` in Zig+C output. Closes #1333.
+- **Anchor**: phi^2 + phi^-2 = 3
+
+
 ## takum32-64-libtakum-parity -- record libtakum C-reference parity witness (Closes #1334)
 
 - **WHERE** (conformance vectors, witness-only): `conformance/vectors/takum32_conformance_v0.json` and `takum64_conformance_v0.json` -- update the `libtakum_c_parity` entry in `witnesses[]` from NOT DONE to results; append one ASCII clause to `format_notes`; refresh both `sha256` values in `INDEX_all_formats.json`. No vector data, no counts, no SSOT, no gen/ edits; SW-axis stays 72/83. WP-18 conformance-integrity-gate (A/B/C/D/D2/E) CLEAN locally.