Corrects the changelog claim that the old guard messages only named env keys:
they named fields too, it was the remediation advice that pointed at env keys.
Records the human approval of the design, the mutation-testing evidence from
the independent verifier, and the same-class gap it found: 14 checks (redis_url
presence, telemetry paths, enum validity) still live only in from_env, while
client.py asserts they were already validated. Deliberately out of scope here.
Implementation confirmed that no invalid GatewaySettings instance can exist, so
the factory test's exception fires while evaluating the argument rather than
inside from_settings. Recorded so the test is not misread as the factory
carrying its own validation.
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'.
iomgaa