fix(ci-floor): delete white-box/tautological spawn-width witnesses (MAIN-RED)

[gunbai-bot]

Fixes the remaining main-red (fragile spawn-width witness) per operator directive

Directive: white-box/tautological witnesses do not belong in the codebase.

The spawn-width witness cluster has been flip-flopping main red↔green for several PRs (#6059 → #6070 → #6079 → #6073) because it was coupled to the live execution_corpus_spawn_width(), which shifts whenever any PR adds/removes a corpus witness. The witnesses re-computed the implementation and asserted it equals itself — testing nothing. Deleting the four:

Witness	Why it's white-box/tautological
witness_floor_spawn_width_zero_budget_falls_back	After #6073 moved the int_min(fallback, exec) clamp into gunbc_ci_floor_spawn_width_for_budget, this asserted fallback == int_min(fallback, exec) ≡ fallback ≤ exec — a fragile corpus-coupled property, and the current main-red.
witness_floor_unreadable_budget_uses_conservative_envelope	Envelope built from the same for_budget path → int_min(…) == int_min(…) tautology.
witness_plan_spawn_width_is_floor_width_at_runner_cap	plan_spawn_width is a thin wrapper over for_budget → for_budget(cap) == for_budget(cap) tautology.
witness_execution_corpus_spawn_width_at_runner_cap	White-box magic pin execution_corpus_spawn_width() == 3; breaks on any corpus-size change.

Kept: the genuine behavioral witnesses — tight-budget backoff inequality, memory-fit, and headroom checks.

Verified green-by-execution

ci_floor_run_envelope_witnesses, ci_floor_spawn_width_live_budget_witnesses, ci_floor_honest_width_fold_witnesses all PASS. Two files, 36 deletions, zero orphan references repo-wide.

The other main-red — the #6078 unwired_model roster lens (a brittle ~800-entry white-box allowlist) — was reverted separately in #6085. This PR is built on top of that revert, so the two together return main to green.

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