Gate 2: execute aggregate Native MIR through QBE

[hiroki23]

Summary

• add nominal aggregate Native MIR for record and tagged-value construction,
  projection, tag tests, direct calls, returns, and block parameters
• verify aggregate shapes, nominal identities, call edges, control-flow edges,
  source origins, and exact scalar semantics before code generation
• lower non-escaping aggregates through caller-owned result storage, borrowed
  parameters, and distinct block transfer/value slots
• zero aggregate storage and copy fixed-size values through explicit system
  memset/memcpy calls
• dynamically check payload projections so malformed MIR fails deterministically
  instead of reading the wrong variant
• build, link, and run a hand-authored record/tagged-value vertical slice through
  pinned QBE 1.3 on Darwin arm64

Verification

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

The vertical slice covers a nested record payload, aggregate call arguments and
results, an aggregate block argument, exhaustive tag dispatch, scalar record
projection, and deterministic invalid payload projection.

Issue #7 remains open for the strict snapshot v3 bridge, real TypeRB source
corpus, and measurements.