self-host P0: MVP compiler core in t27, byte-identical to gen-c, fixpoint on its own source #5981

Part of #5980 (P0).

Scope

A t27 module specs/compiler/core/t27core.t27 that is a compiler: it reads t27 source from a byte buffer, lexes, parses, and writes C to an output buffer.

• Written only in the subset t27c gen-c handles today (global arrays/scalars, scalar fns, while/if/else if/break/continue/return, casts, char literals, and/or, bit ops, shifts). No structs, no strings -- parallel arrays.
• Accepts the same subset it is written in, and rejects everything else with a nonzero result, including the gen-c defect shapes listed in epic: self-host t27c -- rewrite the compiler from Rust into t27 #5980.
• Pure core: compile(len) -> out_len. stdin/stdout live in a ~20-line C driver held by the test harness (plumbing, not part of the language).

Acceptance

• core(fixture) == t27c gen-c(fixture) byte for byte, for every fixture.
• Fixpoint: core(t27core.t27) == t27c gen-c(t27core.t27).
• The generated C of the core passes the core's own test blocks (-DT27_TEST_MAIN).
• Gated by bootstrap/tests/core_selfhost.rs.

Boundary

• specs/compiler/core/t27core.t27
• bootstrap/tests/core_selfhost.rs