self-host P2 slice 3: invariant and bench blocks, byte-identical to gen-c #8011

Part of #5980 (phase P2: "invariant blocks, bench blocks"). This is the third P2 slice; it follows #8000 (structs) and #8007 (enums).

Probed with t27c gen-c on master f2caa84 (t27c lab):

• gen-c writes an invariant whose checks it deems C-constant as _Static_assert(pred, "invariant: NAME"), in an "Invariants" section after the functions. Any other invariant becomes a comment, or a function nothing calls (gen-c: an invariant it cannot write as _Static_assert becomes a comment or an uncalled function (hex/char literal, enum member, static const, call, statement), so C never checks it #8009).
• A bench becomes void bench_NAME(void) { /* bench: NAME */ ... } in a "Benchmarks" section after the tests. Nothing calls it, in C as in Zig.

Today the core refuses both keywords with E_LEX.

This slice

1. specs/compiler/core/t27core.t27 accepts, byte-identical with gen-c:
   • invariant NAME { CHECK; ... } and invariant NAME: CHECK;. A CHECK is an expression, or assert(EXPR), over decimal numbers, true, false and consts gen-c writes as #define, through operators and as. These are exactly the checks gen-c keeps as _Static_assert;
   • an empty invariant, which is a comment in C and unchecked in Zig;
   • bench NAME { STMT* }, including the bench_ prefix rule and the empty-body TODO.
2. New code E_INVARIANT (14) for an invariant gen-c does not check (gen-c: an invariant it cannot write as _Static_assert becomes a comment or an uncalled function (hex/char literal, enum member, static const, call, statement), so C never checks it #8009): a hex, binary or char literal, an enum member, a non-#define const or a var, a call, or a statement.
3. A bench's top-level assignment follows the test rule. Zig binds an unbound name and gen-c leaves it undeclared, so the core refuses it with E_TESTASSIGN (gen-c: a bench body's first top-level assignment is written without a declaration, so cc rejects what the Zig route binds #8010).
4. Fixtures: accept files that build and run, and one refusal per shape.
5. t27core_ddc.t27 is repinned.

Gate: cargo test -p t27c --test core_selfhost on the t27c lab, with the fixpoint, the core tests, every fixture and the corpus differential. The DDC also passes under Zig and t27b.

User Scenarios

• Given a .t27 source file containing invariant NAME { CHECK; ... } with constant-foldable checks, when t27c compiles it in self-hosted mode, then the core accepts the syntax and produces diagnostics byte-identical to gen-c (static-assert invariants pass, non-constant invariants yield E_INVARIANT 14).
• Given a .t27 source file containing bench NAME { STMT* } including empty body and bench_ prefix cases, when t27c compiles it in self-hosted mode, then the core accepts the syntax and follows the test assignment rule (top-level assignment to unbound name yields E_TESTASSIGN).
• Given the updated t27core.t27 and repinned t27core_ddc.t27, when running the self-host test suite, then all core tests, fixtures (accept and one refusal per shape), and the corpus differential pass under cargo test -p t27c --test core_selfhost.

Requirements

• FR-001: The core spec specs/compiler/core/t27core.t27 MUST accept invariant NAME { CHECK; ... } and invariant NAME: CHECK; where CHECK is an expression over decimal numbers, true, false, #define consts, operators, and as, and MUST emit E_INVARIANT (14) for checks containing hex/binary/char literals, enum members, non-#define consts, vars, calls, or statements.
• FR-002: The core spec specs/compiler/core/t27core.t27 MUST accept empty invariant NAME { } (treated as comment/unchecked) and bench NAME { STMT* } with bench_ prefix rule and empty-body TODO handling.
• FR-003: The core spec specs/compiler/core/t27core.t27 MUST apply the test assignment rule to bench top-level assignments: unbound name assignment yields E_TESTASSIGN.
• FR-004: The file specs/compiler/core/t27core_ddc.t27 MUST be repinned to match the updated t27core.t27.
• FR-005: Fixture files MUST exist for each accepted shape (build and run) and one refusal per shape (invariant and bench).

Success Criteria

• cargo test -p t27c --test core_selfhost exits 0 on the t27c lab with the fixpoint, all core tests, every fixture, and the corpus differential passing.
• specs/compiler/core/t27core.t27 compiles without E_LEX on invariant and bench keywords.
• Fixture verification: 5 accept fixtures (build and run) and 2 refusal fixtures (one invariant E_INVARIANT, one bench E_TESTASSIGN) all produce expected exit codes.
• DDC validation: specs/compiler/core/t27core_ddc.t27 repinned and passes under Zig and t27b.

Boundary

• specs/compiler/core/t27core.t27
• specs/compiler/core/t27core_ddc.t27
• specs/tools/trinity/tri/bench.t27
• cli/t27b/bench/bench.py