Add single-source loading and version checks

docs/roadmap/implementation.md

@@ -2,7 +2,7 @@
 
 ## Status and ownership
 
-Status: implementation in progress; Stages 1–6 are complete and later stages are
+Status: implementation in progress; Stages 1–7 are complete and later stages are
 pending.
 
 Audience: a `gpt-5.6-terra` coding agent implementing one stage per prompt, in
@@ -619,7 +619,37 @@ Acceptance:
 
 ## Stage 7 — Assemble single-source loading and schema-version checks
 
-Status: pending.
+Status: complete.
+
+Completion evidence (2026-09-06):
+
+- Added the public generic `Load`, `Definition`, `Options`, `Result`, version,
+  filesystem-context, override, validation-context, and reserved resolution
+  declarations in [`load.go`](../../load.go) and [`validation.go`](../../validation.go).
+  The active lifecycle validates options, parses one source, rejects reserved
+  directives, builds the schema, checks an optional application version, creates
+  fresh defaults, decodes, and captures detached state.
+- Added absent/null/string version handling, non-string rejection, exact/nearest
+  primary and related input locations, safe application reports, warning retention,
+  and generic value-free hook failure only when no explicit error was reported.
+  Defaults run only after version success and exactly once for a continuing load.
+- Populated effective base/default `Metadata` and chronological definition history
+  in [`load_metadata.go`](../../load_metadata.go), with explicit and null state,
+  synthetic default origins, base coordinates, equal-valued input records, and
+  defensive result/source/diagnostic copies. `NoSource` performs the same schema,
+  version, defaults, decode, and projection lifecycle without a source record.
+- Added explicit `options.unavailable` gates for resolution, semantic validation,
+  imports, active profiles, filesystem context, overrides, and secret classification;
+  invalid `NoSource` combinations and `_configkit` directives fail rather than being
+  ignored. The [current single-source contract](../config.md#single-source-loading),
+  package documentation, README, and component inventory own the implemented API.
+- Integration tests cover File/Bytes/Reader/NoSource, schema-version states and
+  warnings, related locations, raw hook-error suppression, fresh-default ordering,
+  metadata agreement and detachment, source lists, unavailable options, reserved
+  directives, and nil results for source/schema/version/decode failures. Verified
+  `go test ./...`, `go test -race ./...`, `go vet ./...`, `go build ./...`,
+  `go mod tidy -diff`, and `git diff --check` with Go 1.26.7 on linux/amd64. No
+  deviations were required; resolution and semantic validation remain later work.
 
 Read: feature [lifecycle](initial.md#resolution-lifecycle),
 [version hook](initial.md#schema-checking-and-secret-classification),