fix(deploy): drop legacy-Python health admission from polymarket shadow gate

[proerror77]

Change contract

The Polymarket shadow gate must not require a healthy legacy Python collector as its baseline-health admission (LEGACY_HEALTH_START_REQUIRED), because the legacy lane is being retired precisely for the degradation that now blocks it (chronic HTTP 429 storms, stale health writes). Parity evidence requirements for the candidate itself are unchanged.

Production evidence (2026-07-31 → 2026-08-01)

• polymarket-reference-collector.service (legacy Python) health: 39+ HTTP Error 429 entries, updated_at == last_success_at frozen for 1.5h+, no health written 15+ min after a clean restart.
• Two consecutive shadow-gate attempts (13:22, 06:27 CST) died at active legacy collector health is not fresh or Gate-admissible (polymarket-raw-ops-shadow-gate.sh:1442), blocking the Rust cutover while 14 market-tape segments backlog.
• Owner decision: legacy historical data quality is not a gate concern; lost data will be recollected, not backfilled.
• Interim production state: the on-host release copy of polymarket-raw-ops-shadow-gate.sh was patched to LEGACY_HEALTH_START_REQUIRED=false (bundle hash recomputed, evidence-preserved) to unblock the in-flight cutover. This PR makes the change official.

Acceptance

• LEGACY_HEALTH_START_REQUIRED defaults to false (or the admission is scoped to structural freshness only), with the incident rationale documented in the script
• Gate policy fixtures updated; test-polymarket-raw-ops-control-plane.sh green
• PR body discloses the host-side patch already applied (evidence parity)

Out of scope

• Weakening any candidate-side gate checks (parity, readback, continuity)
• Legacy Python collector fixes (lane is retired by the cutover)

Runtime control

• Target: polymarket-raw-ops gate/cutover 控制面
• Candidate: cb55ac1d（主机已补丁，本 PR 固化）
• Controller: kimi (authorized by repo owner 2026-07-31)
• Stop rule: cutover 验证失败、Rust collector 健康异常
• Rollback: 按对应 cutover/rollback 脚本回滚到上一个通过 gate 的 digest；证据链保留在 /data/monday/evidence/