Gate 2: decode strict aggregate snapshots

[hiroki23]

Summary

• add an incompatible, strict snapshot v3 schema for nominal record and tagged
  type definitions
• decode Gate 2 aggregate construction, projection, tag test, and payload
  projection instructions into verified Native MIR
• retain snapshot v2 unchanged for the completed Gate 1 scalar checkpoint
• reject unknown fields, unsupported versions/operations, Void storage, invalid
  layouts, and MIR invariant failures deterministically
• add a committed aggregate fixture that decodes, emits, builds, links, and runs
  through pinned QBE 1.3

Verification

• trb fmt --check .
• trb check
• TYPE_RB_NATIVE_ROOT="$PWD" TYPE_RB_NATIVE_QBE=/tmp/qbe-1.3/qbe trb test
  (36 tests, 0 failures)

Issue #7 remains open for the reference TypeRB producer, source differential
corpus, and measurements.