theory: compiler sieve and toolchain theorems as t27 specs (T728+) #6502

Land the compiler sieve and the toolchain theorems as t27 specs (epic #6488).

Three specs under specs/compiler/theory/, all .t27, with the seals t27c seal writes and nothing else:

• toolchain.t27: T728-T734, what each t27c/t27b check certifies (per-test preservation, differential soundness and common mode, vacuity, ratchet monotonicity, DDC model, Futamura equation, the t27a/t27b/t27c ladder), the tool-letter rule and the agent-linking model.
• isa_round_trip.t27: T735-T736, the TRI-27 word does not round-trip for DOT, BIND, BUNDLE2, SACR, BUNDLE3, and a field table that does without moving any existing word.
• sieve.t27: T737+, filters C1-C10 over Zig aarch64, Zig x86_64, cg_clif, Go SSA, QBE, CompCert, LLVM, t27c, t27b with TRUE/FALSE/UNKNOWN cells from opened sources; kills, subsumption, survivors, and the derived golden reference language with t27's status per property.

Every test calls functions, so every assert runs at run time, and each theorem has a negative control.

Refs #6488