[bug] gen-rust: a slice next to a field called len, a slice-typed struct field and a [N]str constant generate Rust that does not compile #11182

Goal

Fix three gen-rust defects the RFC experiment of #11177 found in the lowering of slices and strings, so generated Rust compiles for the forms the Rust back end is meant to carry: a slice parameter next to a struct field called len, a slice-typed struct field, and a [N]str constant.

Why

Found by lane R2 (RFC for #11177, section 6.1, defects D1 to D3), with t27c 0.5.2 built 2026-10-19 13:04. D1: when the module declares a struct field named len, gen-rust leaves t.len as written on a slice parameter instead of t.len(), and rustc fails with E0615; renamed count it compiles. D2: a slice-typed struct field is written as Vec<T> and the constructor takes it by value, so the caller's storage is moved and not borrowed (specs/tri/collections/stack.t27 becomes items: Vec<i64> and empty(storage: Vec<i64>); rustc reports 2 type errors). D3: a [N]str constant element is typed &str, which does not pass where []const u8 is expected (E0308). These sit in the same forms the out-buffer work (#9098, #11109) and every golden port (#11157) rely on.

Boundary

• bootstrap/src/compiler.rs
• specs/tri/collections/stack.t27
• gen/rust/tri/collections/stack.rs

User Scenarios

• Given a module with a struct field len and a function taking a slice, when gen-rust output is compiled with rustc, then it compiles and t.len() is called on the slice.
• Given a struct with a slice-typed field, when gen-rust runs, then the field is a borrowed slice with a lifetime or the spec is refused with a reason, and the caller's storage is not moved.
• Given a [N]str constant passed to a []const u8 parameter, when gen-rust runs, then the output compiles.

Requirements

• FR-001: Each defect MUST have a failing probe (the spec, t27c gen-rust, rustc --crate-type lib) before the fix, kept as a test.
• FR-002: The rules MUST live where the back end's rules live; hand Rust in bootstrap/ MUST NOT grow (own-language net <= 0).
• FR-003: The committed gen/rust outputs MUST be regenerated where they change and the seals re-saved.

Success Criteria

• The three probes compile with rustc and exit 0.
• specs/tri/collections/stack.t27 generates Rust that compiles, with 0 type errors.