D180 — Every typed client-config field is written by Save

Decision. Save emits search_depth, Load strips it from Extra, and a table-driven test asserts that every field of the YAML struct appears in the written file.

Rationale.

• The defect was invisible because a second bug hid it. SearchDepth was read (yamlConfig.SearchDepth) but missing from the struct Save marshals, so it never reached the file from the typed field. It looked fine anyway, because Load did not strip search_depth from Extra, and Save's merge wrote the stale raw value back. A file therefore round-tripped correctly while every programmatic change to the field was silently discarded — including the clamping D162 documents, which could never be persisted, so its warning repeated on every run.
• Both halves had to be fixed together. Fixing only Save leaves Extra shadowing the field, and the merge would keep the stale copy; fixing only Load starts dropping the value from disk entirely.
• Extra is for genuinely unknown keys. A known, typed field living there is a latent conflict, not a redundancy.
• The guard is a test over the struct, not a comment. This defect class is invisible on review — an omission from a literal — so the next one fails a test instead of shipping.

Consequences. None observable for a hand-written config: omitempty means a zero SearchDepth still writes no key, so existing files are not churned.