t27b port: decl_line and parse_int of lower.rs move to specs/tri/t27b/decl_text.t27 #8590

Part of the t27b port (see #6198, epic #8326). t27b must be generated entirely from .t27; this moves one slice of hand-written Rust out of cli/t27b/src/lower.rs.

Problem

These top-level functions of cli/t27b/src/lower.rs are hand-written Rust: decl_line, parse_int. Both read only source text (a declaration's line, an integer literal's value).

How (copy the precedent exactly)

Read #8569, #8350 and #8235 first (gh pr diff 8569 --repo gHashTag/t27). Each one:

• writes the decision in a new spec with test blocks;
• generates gen/rust/tri/t27b/<name>.rs with t27c gen-rust;
• mounts it in lower.rs with #[path = "../../../gen/rust/tri/t27b/<name>.rs"] mod <alias>;;
• calls it from thin glue;
• deletes the hand bodies.
  Keep the function names so the callers stay as they are, or rename them at the call sites in the same file.

Boundary

• specs/tri/t27b/decl_text.t27
• gen/rust/tri/t27b/decl_text.rs
• cli/t27b/src/lower.rs

Requirements

• FR-001: The decisions MUST live in specs/tri/t27b/decl_text.t27 with at least one test block per moved function. Each test MUST call the function by name.
• FR-002: gen/rust/tri/t27b/decl_text.rs MUST be the unedited output of t27c gen-rust specs/tri/t27b/decl_text.t27 (L2).
• FR-003: The PR MUST be net <= 0 in hand-written lines across all files (specs/policy/own_language.t27, check_budget, policy: hand-written code in any language but t27 may only shrink (Closes #8227) #8236). The moved bodies are deleted, and glue is thin.
• FR-004: cargo test -p t27b MUST still pass, and t27b's corpus verdicts MUST NOT change. This is a pure port.
• FR-005: The steward adds the new spec's ledger row and seal at merge; do not edit docs/reports/t27b_expectations.json or .trinity/seals/ in this branch.

Success Criteria

• grep -c '^fn decl_line' cli/t27b/src/lower.rs prints 0
• grep -c '^fn parse_int' cli/t27b/src/lower.rs prints 0
• grep -c 'decl_text.rs' cli/t27b/src/lower.rs prints 1
• t27c spec-status specs/tri/t27b/decl_text.t27 prints IMPLEMENTED

User Scenarios

• Given the t27b compiler's lower.rs contains hand-written decl_line and parse_int functions, when the port is complete, then those functions are implemented in specs/tri/t27b/decl_text.t27 and the generated gen/rust/tri/t27b/decl_text.rs is mounted in lower.rs with thin glue.
• Given the new spec specs/tri/t27b/decl_text.t27 with test blocks for each moved function, when t27c gen-rust runs, then gen/rust/tri/t27b/decl_text.rs is produced as the unedited L2 output and t27c spec-status specs/tri/t27b/decl_text.t27 reports IMPLEMENTED.
• Given the PR deletes the hand-written function bodies from lower.rs and adds the mount + glue, when cargo test -p t27b runs, then all tests pass and t27b's corpus verdicts remain unchanged.