Add Gate 1 QBE vertical slice

[hiroki23]

Summary

• add strict snapshot v2 decoding and the heap-free scalar Gate 1 MIR verifier
• emit QBE 1.3 IL from TypeRB and link Darwin arm64 executables through the system toolchain
• cover direct calls, block arguments, branches, loops, scalar arithmetic, static output, and checked Integer failure
• record the disposable QBE target decision and move records/tagged values to Gate 2

Verification

• trb fmt --check .
• trb check
• TYPE_RB_NATIVE_ROOT="/Users/fujita-h/trb/worktrees/type-rb-native/gate-one" TYPE_RB_NATIVE_QBE=/tmp/qbe-1.3/qbe trb test (16 tests, 0 failures)

Gate 1A foundation for #4; Gate 1B and measurements remain.