Model §7 self-host fixpoint condition in self_host.dag

[briansrls]

Summary

• Model the self-host fixpoint condition natively in src/v2/compiler/self_host.dag: what stage1 and stage2 are (first/second self-compile emissions), the equality relation over canonical_node_emission (content_hash digests), and when the snake closes.
• Add SelfHostFixpointCondition as the single-authority bundle for representation + stages; refactor self_host_snake_closes and wire fixed_point_scaffold through self_host_fixpoint_witness.
• self_host_fixed_point_validate remains fail-closed (self_host_runner_not_realized) until Stage C runner lands — honest contract per Track B.
• Add 16 discriminating floor witnesses in fixed_point_test.dag (equal/perturbed fixtures, scaffold, phase grounding); fix test shadowing infinite-recursion bug on equality-representation witness.

Test plan

• ./target/debug/claim_batch --source-root src/v2 --entry src/v2/test/claim/self_host/fixed_point_test.dag --function <each of 16 test fns> — all PASS

[gunbai-bot]

Addressed the Opus REQUEST_CHANGES findings in 14a93cf:

1. Stage1/Stage2 identical records + pinned phase — removed SelfHostStage1/SelfHostStage2 and the constructor-pinned phase field. Stage1 and stage2 are now positional SelfHostStageEmission values inside SelfHostFixpointEqual; the ordinal is the field name, not a redundant sentinel.

2. SelfHostFixpointCondition ≡ SelfHostFixpointEqual nicknaming — collapsed to one record SelfHostFixpointEqual { over, stage1, stage2 }. Dropped the FixedPointEqual { fixpoint: … } wrapper; FixedPointCheck = Witness<SelfHostFixpointEqual> directly.

3. SelfHostEqualityRepresentation one-field wrapper + tautology — replaced with data self_host_equality_authority: Symbol = ^canonical_node_emission (single authority). Removed self_host_equality_representation_is_canonical_node_emission() and the constructor-pinned phase tests. The remaining authority witness (self_host_fixpoint_equal_carries_equality_authority) reads the data binding, not a value the constructor just wrote.

Discriminating equal/perturbed witnesses unchanged and green locally via claim_batch.

— sent from swift-ibex-119