Gate 2: establish heap-free aggregate layouts

[hiroki23]

Summary

• reframe TypeRB Native as a sustained implementation effort while retaining
  experimental API and artifact stability
• stage Gate 2 around heap-free records and tagged values, moving dynamic
  allocation concerns to Gate 3
• pre-register Gate 2 correctness and performance targets in issue Gate 2: heap-free aggregate value model #7
• add nominal aggregate definitions and deterministic Darwin arm64 layout
  computation for records and tagged values
• reject recursive, missing, duplicate, oversized, or otherwise invalid layouts
  with deterministic diagnostics

Verification

• trb fmt --check .
• trb check
• TYPE_RB_NATIVE_ROOT="$PWD" trb test (26 tests, 0 failures)

This is the first Gate 2 implementation slice. It establishes the backend-neutral
layout contract before Native MIR and QBE aggregate lowering are added.

Closes no issue; #7 remains open until the complete Gate 2 checkpoint is
measured and reported.