epic: t27b in t27 -- port the backend and its tooling from Rust/Python into .t27 #6198

Refs #6063 #5980

Problem

Owner, 2026-10-05: t27b must be written in .t27. Today none of it is:

• cli/t27b/src/*.rs: 8247 lines of hand-written Rust, and it mounts bootstrap/src/compiler.rs through #[path]. Its tests in cli/t27b/tests/*.rs add 2865 more.
• scripts/tri_loop/t27b.py: 894 lines of Python (tri t27b status|doctor|delta|ratchet).
• contrib/railway/t27b-lab/lab.py: 608 lines of Python (the Railway lab).
  (Line counts are from wc -l on master c532fca.)

Proposal

• Spec first: each t27b stage (front, lower, ir, codegen/a64, macho) gets a .t27 spec with test blocks, generated with t27c gen-rust. The first slice to port is the smallest pure one. The steward tool's pure functions (honest, delta, ratchet) are the candidates, because they have fixtures already.
• This rides on the self-host epic epic: self-host t27c -- rewrite the compiler from Rust into t27 #5980: whatever gen-rust cannot yet express gets filed there, not worked around by hand.

Done when

• Every t27b source file is generated from a .t27 spec. The hand-written remainder is listed in AGENTS.md, and that list only shrinks.

Boundary

• bootstrap/src/compiler.rs
• contrib/railway/t27b-lab/lab.py
• scripts/tri_loop/t27b.py

User Scenarios

• Given the t27b steward tool's pure functions (honest, delta, ratchet) have existing fixtures, When a developer writes a .t27 spec for one of these functions and runs t27c gen-rust, Then the generated Rust code passes the existing fixture tests without hand-written Rust.
• Given the t27b CLI entry points (status|doctor|delta|ratchet) are invoked via tri t27b, When the spec for a command is complete and generated, Then tri t27b <command> produces identical output to the current Python implementation.
• Given the Railway lab (contrib/railway/t27b-lab/lab.py) exercises t27b stages, When all stages have .t27 specs with test blocks, Then lab.py can be replaced by a .t27 spec that drives the same verification.

Requirements

• FR-001: Each t27b stage (front, lower, ir, codegen/a64, macho) MUST have a corresponding .t27 spec file containing test blocks.
• FR-002: The steward tool's pure functions (honest, delta, ratchet) MUST be the first stages ported to .t27 specs with t27c gen-rust output.
• FR-003: Generated Rust code from .t27 specs MUST pass all existing fixture tests in cli/t27b/tests/*.rs without modification.
• FR-004: The Python CLI in scripts/tri_loop/t27b.py MUST be replaceable by generated code from .t27 specs such that tri t27b status|doctor|delta|ratchet behaves identically.
• FR-005: The Railway lab in contrib/railway/t27b-lab/lab.py MUST be expressible as a .t27 spec that drives the same stage verification.
• FR-006: Any capability not yet expressible in gen-rust MUST be filed as a blocker in epic epic: self-host t27c -- rewrite the compiler from Rust into t27 #5980 rather than worked around with hand-written code.
• FR-007: The hand-written remainder in AGENTS.md MUST only shrink as stages are ported; no new hand-written files MAY be added to t27b.
• FR-008: The mounted bootstrap/src/compiler.rs via #[path] in cli/t27b/src/*.rs MUST be eliminated once the corresponding stage has a .t27 spec.