t27c: six silent misreads -- a colon inside a field type, and @as to a slice type #5968

tri misread --list (after #5947) reports 41 spec-shape pairs. 35 are refused by typecheck; 6 are silent: the spec parses, typechecks and generates output its backend cannot parse.

Two compiler defects behind them:

1. A : inside a struct field's type (after removing :: paths). A field with no , after it, whether the comma is missing or sits inside a trailing # comment, swallows the declarations after it into its type: pub identity: Stringsacred_score:Floatgeneration:Int. Two fields disappear, and parse and typecheck accept it. A map type [K: V] has the same footprint (Vec<str:str>). The corpus holds 144 fields with a colon in their type; 121 are :: paths, and 23 carry a lone :.
2. @as(T, x) with a slice or array type T. In argument position the parser reads []u8 as an empty array literal, and every backend loses the type: gen-zig @as(.{}, x), gen-c ({ 0 })(x), gen-rust (x as Vec<>). [4]u8 becomes @as(.{ 4 }, x) / Vec<4>. 3 specs use it (wp18, jit/jit, tri/pipeline/builder).

Plan: typecheck refuses both and names the line; parse is unchanged, so generation is unchanged. Rewrite the six specs, plus the builder test line, in t27 where t27 can say it, and ledger the rest. Report the silent count before (6) and after.

Boundary

• bootstrap/src/compiler.rs