Add safe historical configuration explanations

docs/config.md

@@ -141,10 +141,60 @@ translated to a fixed safe error. There is no unredacted rendering option and
 ConfigKit performs no implicit logging.
 
 `Result.Config` is the explicit trusted boundary and contains ordinary typed values,
-including secrets. Metadata, histories, diagnostics, and load errors contain no raw
-configuration values. Applications must still keep source labels, resolution
-reasons, and diagnostic messages value-free because ConfigKit cannot infer and
-sanitize secrets embedded in arbitrary application prose.
+including secrets. Metadata records, diagnostics, and load errors contain no
+configuration values. Explanation snapshots deliberately expose non-secret
+configuration history after applying the redaction contract below. Applications
+must still keep source labels, resolution reasons, and diagnostic messages value-free
+because ConfigKit cannot infer and sanitize secrets embedded in arbitrary application
+prose.
+
+### Historical-value explanation
+
+`Result.Explain(path)` returns a detached `Explanation` for any known effective or
+historical path. Its effective value and each chronological before/after value are
+`ValueSnapshot` records containing deterministic YAML text rather than mutable Go or
+parser values. `Present=false` with empty YAML means absence or deletion; explicit
+null is present and renders as `null`. The history starts with the canonical typed
+application-default or Go-zero baseline, then includes only relevant base sources in
+traversal order, the selected profile, supplied CLI records, and audited net
+resolution operations. Explicit equal writes remain visible.
+
+Base, profile, and CLI history preserves the policy-validated input representation,
+including a superseded value that could not have decoded into the final Go field.
+Baseline, effective, and resolution values use canonical typed projection. Mapping
+queries reconstruct each source or override contribution, while sequences remain
+atomic: a descendant history identifies its presence before and after the containing
+sequence replacement without implying element-wise merging. Removed paths remain
+explainable; unknown and malformed paths return false.
+
+Every snapshot applies the same historical tag, predicate, and recorded-dependency
+sensitivity closure as effective rendering before YAML is exposed. A redacted scalar
+is the quoted marker `'[REDACTED]'`, and a container snapshot has `Redacted=true` if
+it contains a masked descendant. Typed `Secret` snapshots contain only the configured
+environment name. `SecretReference` reports that name only for a successfully
+resolved effective reference, and is omitted when its path is classified. Credential
+values are never retained in explanation history.
+
+`Result.WriteExplanation(output, path)` renders the safe structured result with the
+canonical JSON-pointer path, canonical-versus-input representation labels, effective
+source, composite locations, chronological values, resolution reasons and
+dependencies, and successful secret-reference lookup attribution. Control characters
+in paths, source identifiers and labels, reasons, dependencies, and reference names
+are escaped. An unknown path returns a fixed safe error before writing anything;
+writer errors and short writes are returned and may leave partial output. There is no
+unredacted mode.
+
+Retained history consists of immutable source applications and audited subtree
+deltas, rather than a full tree copy per field. Shared nodes count once toward a
+4,000,000-node retained-history limit; exceeding it fails loading with
+`explain.limit_exceeded`. Replaying one requested path is performed on demand, and
+the returned YAML strings and nested records cannot mutate later explanations.
+
+Application commands should render the separate `SecretsReport` immediately after
+an explicit `LoadSecretsDir` bootstrap, then use `WriteDiagnostics`,
+`WriteEffective`, or `WriteExplanation` on the later load result as appropriate.
+ConfigKit does not infer bootstrap outcomes from a secret reference and does not
+bundle these independent reports into a command framework.
 
 ## Typed secret references