t27b: move the type-text rules of lower.rs (type_base, close_of_open, type_construct's shape and more) to specs/tri/t27b/type_text.t27 #8229

Port slice of #6198 (t27b written in t27), under #6063 and #5980.

What

cli/t27b/src/lower.rs lowers the t27c AST to t27b's IR. A type reaches it as the source spelling the t27c parser keeps (?*const [4]u8, [T; N], gf16::GF16). Several of the rules that read that spelling ask nothing of the rest of the file, and are hand-written Rust today:

• close_of_open: the ] that closes a leading [;
• type_base: the element name under ?, &, *, const, a postfix ? and the array and slice forms;
• zig_type_name and undeclared_field_type: a struct field type the reference emits as a name Zig cannot resolve;
• is_scoped_gf16: m::GF16 as u16;
• returns_value: a return type that is a value;
• the shape decision of type_construct: the construct a refused type is counted under.

This issue moves them to specs/tri/t27b/type_text.t27.

Plan

• The spec has test blocks whose expected values come from the hand-written Rust on master. Trimming follows Rust's str::trim, including the non-ASCII White_Space chars, so the port is exact.
• t27c gen-rust writes gen/rust/tri/t27b/type_text.rs, which is mounted with #[path] in lower.rs as tt. The hand bodies are deleted. Glue keeps only the lookups in the file's declarations (type_decls, the declared set) and the strings refusals are built from.
• Evidence: seal, t27c test-report with 0 vacuous, tri mutate spec, a differential of the hand functions against the generated ones, cargo test -p t27b, and signed corpus receipts of base and head (verified: a lane verdict over two corpus receipts, and lab receipts for a requested commit (Closes #7672) #7686).

Done when

• lower.rs loses those hand-written bodies, its hand glue stays within master's check_budget caps, and the net change in hand lines under cli/t27b/ is negative.
• The corpus receipts show no file changed its verdict or its t27b asm digest, apart from the new spec itself.
• The AGENTS.md t27b remainder line is re-measured.