fix(gen-rust): lower ExprCast -- emit '(operand as target)' instead of '()' (Closes #1314) (#1320)

Add Expr::Cast arm in expr_to_rust (bootstrap/src/compiler.rs), mirroring
the gen-verilog arm at compiler.rs:4941. Before this fix, ExprCast fell
through to the default '_ => "()".to_string()' branch and generated
empty-tuple stubs in Rust output, silently corrupting any T27 spec using
bit-width casts.

Also update docs/NOW.md with an exprcast-rust-emitter entry (required by
the NOW Sync Gate).

Closes #1314 (gen-rust half; gen-c and gen-zig follow-ups tracked
separately). Unblocks gHashTag/tri-net#33 T27-first wire flip.

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

Co-authored-by: Perplexity Computer <agent@perplexity.ai>

Author: gHashTag

bootstrap/src/compiler.rs

@@ -8169,6 +8169,24 @@ impl RustCodegen {
                 s.push('}');
                 s
             }
+            NodeKind::ExprCast => {
+                if node.children.is_empty() {
+                    "()".to_string()
+                } else {
+                    let operand = Self::expr_to_rust(&node.children[0]);
+                    let target = node
+                        .extra_type
+                        .split('[')
+                        .next()
+                        .unwrap_or("")
+                        .trim();
+                    if target.is_empty() {
+                        operand
+                    } else {
+                        format!("({} as {})", operand, target)
+                    }
+                }
+            }
             _ => "()".to_string(),
         }
     }

docs/NOW.md

@@ -2,6 +2,12 @@
 
 Last updated: 2026-07-04
 
+## exprcast-rust-emitter -- lower ExprCast in gen-rust backend (Closes #1314)
+
+- **WHERE**: `bootstrap/src/compiler.rs` -- added `Expr::Cast { operand, target }` arm in `expr_to_rust`, emits `format!("({} as {})", operand, target)` mirroring the existing gen-verilog arm at compiler.rs:4941. Before this fix, ExprCast in `expr_to_rust` fell through to the default `_ => "()".to_string()` branch, producing empty-tuple stubs in generated Rust and silently corrupting any T27 spec that uses bit-width casts (bit-shift lowering, width promotion, etc.). +18 / -0 lines. Isolated repro filed as #1314 with a 5-line spec showing the empty-tuple emission.
+- **Why**: gen-rust is one of three lowering targets (Rust, Verilog, C). The Verilog and C emitters had ExprCast; the Rust emitter did not. This closes the gen-rust half of #1314; gen (Zig) and gen-c follow-ups tracked separately. Unblocks the tri-net T27-first wire flip (gHashTag/tri-net#33), where specs/wire.t27 needs the fixed emitter to regenerate gen/rust/wire.rs without hand-patching.
+- **Anchor**: phi^2 + phi^-2 = 3
+
 ## bcd-bitexact-promotion -- promote bcd -> strict bitexact (2-digit packed instance) (Closes #1321)
 
 - **WHERE** (conformance vectors): `conformance/vectors/bcd_conformance_v0.json` promoted from `structural` to `bitexact` in `INDEX_all_formats.json` (bitexact_packs 69 -> 70, structural_packs 14 -> 13; bcd.kind structural -> bitexact, n_vectors 0 -> 100). Fixes ONE concrete instance: 2-digit packed BCD, 8-bit word bcd_in[7:4]=tens, bcd_in[3:0]=ones, decode value = tens*10 + ones (valid 0..99). Generic variable-width BCD (u4_per_digit) stays noted in catalog.instance/format_notes; only this fixed-width instance carries a bit-precise round-trip claim.