docs: design settings invariant guards on every construction path

Guards for the three cross-field invariants (source timeout vs lease TTL,
stall window vs max TTFT, probe TTL vs slowest timeout) only ran inside
from_env, so the from_settings path could build a GatewaySettings that
violates the class's own documented invariants. Design moves all of them
plus a non-empty sources check into __post_init__ as _validate_* methods,
matching the existing frozen dataclasses in types.py.

Covers two defects left by PR#1: probe_ttl_s was never moved, and an empty
sources tuple leaked a bare 'max() arg is an empty sequence'.

research-wiki/index.md

@@ -1,13 +1,15 @@
 # Research Wiki 索引
 
-> 自动生成，更新时间：2026-07-22 14:36 UTC
+> 自动生成，更新时间：2026-07-30 03:44 UTC
 
-## design (10)
+## design (12)
 - [2026-07-20-m1-core-design](designs/2026-07-20-m1-core-design.md) `design:2026-07-20-m1-core-design`
 - [2026-07-20-m2-distributed-design](designs/2026-07-20-m2-distributed-design.md) `design:2026-07-20-m2-distributed-design`
 - [2026-07-21-m25-resilience-design](designs/2026-07-21-m25-resilience-design.md) `design:2026-07-21-m25-resilience-design`
 - [2026-07-21-m3-ocr-design](designs/2026-07-21-m3-ocr-design.md) `design:2026-07-21-m3-ocr-design`
 - [2026-07-22-m4-migration-design](designs/2026-07-22-m4-migration-design.md) `design:2026-07-22-m4-migration-design`
+- [2026-07-29-settings-invariant-guards-design](designs/2026-07-29-settings-invariant-guards-design.md) `design:2026-07-29-settings-invariant-guards-design`
+- [GatewaySettings 跨字段不变量守卫的生效范围](designs/settings-invariant-guards.md) `design:settings-invariant-guards`
 - [M1 核心里程碑设计:公共签名冻结与治理栈落地](designs/m1-core-design.md) `design:m1-core-design`
 - [M2 分布式:Redis 治理后端+背压+Postgres 遥测+pricing+Embedding+压测 harness](designs/m2-distributed.md) `design:m2-distributed`
 - [M2.5 治理韧性: 半死源隔离与健康感知调度](designs/m25-resilience.md) `design:m25-resilience`