0348 — AlterTopic takes a sparse pointer-per-field patch; nil means leave alone

Context. RegisterTopic’s topic.Config uses zero-means-default semantics — WithDefaults resolves unset fields before the row is written. That works for create but not for patch: a value field cannot distinguish “set RetentionTTL to 0” from “did not mention RetentionTTL,” and zero is a real, often destructive setting here (RetentionTTL 0 = keep forever; a tuned JanitorSweepBatchSize silently reset). Dated approximately; built across July 2026.

Decision. topic.AlterConfig is pointer-per-field: nil = leave alone, non-nil = set, including an explicit zero. Its Validate is stricter than Config.Validate on the > 0 fields, because there is no WithDefaults to absorb a zero — it would land in the row. An all-nil patch is a usage error, not a silent no-op.

Consequences. A forgotten field no-ops instead of clobbering a live value to zero: the same operator mistake fails safe under a sparse patch and fails destructively under full-replace. Rejected: reusing topic.Config / a full-replace struct — the operator must restate every field to change one, and any field they forget becomes zero and lands. Rejected: “require every field” — not enforceable in Go, since a missing struct field is a silent zero, not a compile error; the destructive version is what the language hands you by default.

Superseded by [0518]: topic config stops being operator-writable at all, so AlterTopic and its patch shape are deleted rather than reshaped.