duplicate-bodies red on master after #7747: coerce_plan.t27 copies int_cast_plan's widens and icp_bits #7748

Part of #6063. Follows #7747 (#7740), which auto-merged at 09:24Z with this check red.

duplicate-bodies (tools/dupe_scan.py) fails on master 1d9c596:

a body written under cp_bits is now copied [2] time(s) and was in no ledger
a body written under widens is now copied [2] time(s) and was in no ledger

specs/tri/t27b/coerce_plan.t27 wrote its own widens (Zig's lossless integer rule) and a test helper cp_bits, and specs/tri/t27b/int_cast_plan.t27 already has both (widens, icp_bits).

Fix: reuse them, not bless the copies.

• coerce_plan.t27 adds use tri::t27b::int_cast_plan; and calls widens, icp_bits and icp_signed.
• t27c gen-rust resolves the use and copies what it needs into gen/rust/tri/t27b/coerce_plan.rs.
• The plan's KEEP, WIDEN, FOLD and why are also int_cast_plan's names. They become INT_KEEP, INT_WIDEN, INT_FOLD and compare_why, so neither plan shadows the other in the generated file.
• The glue in cli/t27b/src/lower.rs changes 3 lines, for the new names only.

User Scenarios

• Given the duplicate-bodies check fails on master because coerce_plan.t27 duplicates widens and cp_bits from int_cast_plan.t27, when the spec is changed to use tri::t27b::int_cast_plan and call its widens, icp_bits, and icp_signed, then tools/dupe_scan.py passes with no duplicate-body complaints.
• Given t27c gen-rust resolves the use and emits renamed constants (INT_KEEP, INT_WIDEN, INT_FOLD, compare_why) into gen/rust/tri/t27b/coerce_plan.rs, when the generated code is compiled, then no symbol shadows the int_cast_plan equivalents.
• Given cli/t27b/src/lower.rs is updated for the three new names, when the CLI crate builds, then it compiles without errors.

Requirements

• FR-001: specs/tri/t27b/coerce_plan.t27 MUST add use tri::t27b::int_cast_plan; and replace its local widens, cp_bits, and related logic with calls to int_cast_plan::widens, int_cast_plan::icp_bits, and int_cast_plan::icp_signed.
• FR-002: t27c gen-rust MUST resolve the use and emit the plan constants as INT_KEEP, INT_WIDEN, INT_FOLD, and compare_why in gen/rust/tri/t27b/coerce_plan.rs so they do not shadow int_cast_plan names.
• FR-003: cli/t27b/src/lower.rs MUST change exactly three lines to reference the new generated names.
• FR-004: Running tools/dupe_scan.py MUST exit 0 with zero duplicate-body warnings for widens and icp_bits/cp_bits.

Success Criteria

• tools/dupe_scan.py exits 0 and prints no "copied [2] time(s) and was in no ledger" lines for widens or icp_bits/cp_bits.
• t27c gen-rust completes successfully and gen/rust/tri/t27b/coerce_plan.rs contains INT_KEEP, INT_WIDEN, INT_FOLD, and compare_why (no un-prefixed KEEP, WIDEN, FOLD, why from this plan).
• cargo build -p t27b (or the project's equivalent CLI build command) exits 0.
• The test suite for the t27b CLI passes (at least 5 tests) after the changes.

Boundary

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