t27core: for over one range and switch as a value, byte-identical to gen-c #7429

Part of #5980.

The self-hosted core (specs/compiler/core/t27core.t27) should accept for over one range and switch as a value, and write the same C bytes as t27c gen-c, once gen-c lowers them (#7331, closing #7307 and #7308).

A for (a..b) |i| { } becomes for (size_t i = a; i < b; i++) {, and a |_| capture names the counter __t27_i. C reads the end on every pass while Zig reads it once, so the core takes a for only when the end calls nothing and the body can neither assign a name the end reads nor, when the end reads a module var, make any call. Two ranges, a for-each over an array (#7420) and ..= are refused.

A switch becomes a chain of ternaries, (x == 1) ? (10) : 0. gen-c loses several shapes (#7419), so the core takes a switch only as the whole value of a return, a local initializer or an assignment, or as an arm value. The subject must call nothing, each label must be a number without _ separators (an optional - is allowed), and else must come exactly once and last. Everything else is refused.

The fixpoint and the DDC pin move with the core. New accept fixtures cover both shapes and run under C, and the refusals name the gen-c defect each shape avoids.

Boundary

• specs/compiler/core/t27core.t27

User Scenarios

• Given a t27 source with for (0..10) |i| { sum += i; } where the end expression calls nothing and the body doesn't assign names read by the end, When compiled by t27core, Then the emitted C is byte-identical to t27c gen-c output for the same construct.
• Given a t27 source with return switch (x) { 1 => 10, else => 0 }; where the subject calls nothing, labels are plain numbers, and else is last, When compiled by t27core, Then the emitted C is a ternary chain (x == 1) ? 10 : 0 byte-identical to t27c gen-c output.
• Given a t27 source with a refused shape (two ranges, for-each over array, ..=, switch subject that calls, non-numeric label, missing or misplaced else), When compiled by t27core, Then compilation fails with a message naming the gen-c defect that shape avoids.

Requirements

• FR-001: t27core MUST accept for (a..b) |i| { body } only when the end expression b calls no function, the body assigns no name that b reads, and if b reads a module variable the body makes no calls.
• FR-002: t27core MUST lower accepted for (a..b) |i| { body } to C for (size_t i = a; i < b; i++) { body } with |_| capture naming the counter __t27_i, byte-identical to t27c gen-c output.
• FR-003: t27core MUST refuse two-range for, for-each over array, and ..= inclusive range with a diagnostic naming the gen-c defect each avoids.
• FR-004: t27core MUST accept switch as a value only when it is the whole value of a return, a local initializer, an assignment, or an arm value; the subject calls nothing; each label is a plain integer literal (optional - allowed, no _ separators); and else appears exactly once and last.
• FR-005: t27core MUST lower accepted switch to a ternary chain (subject == label1) ? value1 : (subject == label2) ? value2 : ... elseValue byte-identical to t27c gen-c output.
• FR-006: t27core MUST refuse all other switch shapes with a diagnostic naming the gen-c defect each avoids.
• FR-007: New accept fixtures for both shapes MUST be added and MUST pass under C execution.
• FR-008: The fixpoint and DDC pin MUST be updated to match the new core.

Success Criteria

• t27c check specs/compiler/core/t27core.t27 exits 0
• t27c build specs/compiler/core/t27core.t27 exits 0 and produces a binary that compiles the accept fixtures
• specs/compiler/core/fixtures/accept/for-range-one.t27 compiles and runs under C with exit 0
• specs/compiler/core/fixtures/accept/switch-value.t27 compiles and runs under C with exit 0
• specs/compiler/core/fixtures/refuse/for-range-refusals.t27 fails to compile with diagnostics naming gen-c defects (5 tests)
• specs/compiler/core/fixtures/refuse/switch-value-refusals.t27 fails to compile with diagnostics naming gen-c defects (7 tests)
• Fixpoint rebuild t27c fixpoint exits 0
• DDC pin update t27c ddc-pin exits 0