t27b: move the source-text rules of lower.rs (parse_int, char_literal, int_lit_width, header_line and more) to specs/tri/t27b/source_text.t27 #8248

Port slice of #6198 (t27b written in t27), under #6063 and #5980. It follows #8229 (merged), whose specs/tri/t27b/type_text.t27 it reuses for trimming.

What

cli/t27b/src/lower.rs reads source text that the t27c parser keeps in the AST, with hand-written Rust:

• parse_int: an integer literal's text, decimal or 0x / 0X, 0o, 0b, with _ separators;
• char_literal: a char literal's text, with the escapes Zig takes;
• int_lit_width: the width t27c's Zig backend pins on an untyped var of an integer literal;
• header_line: the line of a <keyword> <name> header in the source, for a node the parser built from text;
• is_prose_clause: the <word>: label of a prose clause;
• the element split of text_lit: an array literal whose elements the parser kept as one text.

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

Plan

• The spec has test blocks whose expected values come from the hand-written Rust on master. The integer parser follows Rust's u128::from_str_radix, so its optional + and its overflow behave as before.
• t27c gen-rust writes gen/rust/tri/t27b/source_text.rs, which is mounted with #[path] in lower.rs as st. The hand bodies are deleted, and the old signatures stay as one- or two-line glue.
• 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.