feat(frontend): make the front half of the compiler field-generic

[BornPsych]

Summary

The front half of the compiler, from source text to the monomorphized AST, is now correct for more than one prime field: an integer is never carried through the field where it might not fit, a value outside the field is an error rather than a reduction, and stdlib items that only work on bn254 are gated to it. bn254 stays the default and fully green. The goldilocks feature of noirc_driver selects Goldilocks (p = 2^64 − 2^32 + 1) in every crate below it, and the Goldilocks CI lane now runs the ABI, frontend and stdlib checks under that build.

The rules, in one place:

• x as Field is the identity on the value and is defined only for source types whose every value lies below p. An unsigned type that can hold a value at or above p (u64 and u128 under Goldilocks) is refused at type check; the escape hatch is an explicit narrowing cast. Signed sources stay a type error.
• A Field literal or a comptime Field result must lie in [0, p); nothing reduces a value into the field.
• Integer literals up to u128 stay exact in the monomorphized AST under every field.
• At the entry point, one field element carries one scalar: an integer is accepted iff it is in the range of its type and its fixed-width bit pattern is below p.

Changes

• acir_field: to_i128 reads the shorter of an element's two spellings on the field's own byte width (8 bytes under Goldilocks, 32 under bn254), with a round-trip test generic over the field; assert_unique_feature! covers goldilocks.
• Parser and AST: impl blocks and trait impls keep their attributes. The formatter prints them and the LSP remaps their locations; function-only attributes on an impl are now a parse error instead of being dropped silently.
• #[field(..)] gates impl blocks, impl methods, trait impls (whole blocks), inline modules, mod x; declarations and comptime-generated impls. A gated-out item is dropped at definition collection before anything about it is interned, so two same-named items gated to different fields coexist and a gated-out mod x; needs no file.
• Comptime casts evaluate on the source value as a BigInt: an integer target takes the two's complement pattern at the target width and reads it by the target's signedness; a Field target takes the source's own-width pattern exactly and is an error if it is not below p.
• ABI input parsing: scalar_to_field applies the one-element rule for both formats and for quoted and native spellings, and noirc_abi forwards the goldilocks feature.
• Type checker: IntegerTypeExceedsField refuses as Field from an unsigned type whose width reaches the field's. The monomorphizer re-checks a cast whose source type was still a type variable at type check.
• stdlib: From<u64> and From<u128> for Field, Hash for u64/u128/i64, and the wrapping arithmetic on those types (implemented through the field) are gated to bn254. The AsPrimitive<Field> impls are generated only for widths below the modulus. The driver test stdlib_field_casts pins that the stdlib never casts a wide integer to Field under either build.
• CI: the Goldilocks lane runs acir_field, noirc_abi, the frontend suite minus six bn254-pinned tests, and the driver's stdlib checks.

Each place where the feature flag stands in for a runtime field selection carries a TODO naming the follow-up: a runtime FieldConfig, struct and trait gating with a negated form and a width predicate, and native wrapping arithmetic on wide types.

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