Complete the TypeRB Native Gate 0 boundary

[hiroki23]

Summary

• establish TypeRB-owned self-hosting as the long-term implementation and bootstrap boundary
• define the cost-conscious backend sequence: QBE first, TinyGo only as an optional baseline calibration, and C/other backends only for measured questions
• implement Gate 0 entirely in TypeRB with strict snapshot decoding, snapshot verification, Native MIR lowering, and independent MIR verification
• preserve source identifiers, paths, and spans through lowering
• add deterministic diagnostics, valid and invalid fixtures, pinned-compiler CI, and a repository development skill

Gate 0 evidence

• trb fmt --check .
• trb check
• TYPE_RB_NATIVE_ROOT="$PWD" trb test (9 tests, 0 failures)
• committed fixture corpus covers valid lowering, malformed JSON, unknown fields, unsupported versions, unknown source origins, and undefined values
• repository skill passes the skill creator validator

Explicit boundary

This PR stops at Gate 0. It does not emit machine code, add a reference-compiler snapshot producer, implement QBE lowering, or make performance claims. Those belong to Gate 1 after review of the Gate 0 result.

Closes #2