Add private sequence-item placement

docs/roadmap/implementation.md

@@ -1,6 +1,6 @@
 # Implementation Plan: Sequence Placement and Composition
 
-Status: implementation in progress; stages 1–6 completed.
+Status: implementation in progress; stages 1–7 completed.
 
 ## Purpose and authority
 
@@ -451,6 +451,22 @@ Scope: placement primitives and engine per-input options; not public exposure.
 Validation: placement/accounting/option tests and `./scripts/check.sh`.
 Completion: the private operation places a mapping once with correct ownership.
 
+Completion evidence: added private placement modes to engine options, load
+requests, and source records, with preflight rejection of unknown modes and
+item placement at an empty destination. A completed mapping can now be wrapped
+in one singleton sequence at its destination; nil payloads remain absent and
+empty mappings remain real items. Wrapper accounting includes both destination
+mapping nodes and the added sequence depth/node, with limits checked before
+allocation. Focused tests cover mapping zero-value equivalence, nested paths,
+empty and absent payloads, invalid modes/destinations, and exact/exceeded
+allowances. These checks passed with Go `go1.26.9 linux/amd64`:
+
+- `GOTOOLCHAIN=local GOPROXY=off go test ./internal/engine -run 'Destination|Import|Mounted|PlaceDocument|Options|Layered' -count=1`
+- `GOTOOLCHAIN=local ./scripts/check.sh`
+- `git diff --check`
+
+No environmental limitation affected validation; checks ran on Linux/amd64.
+
 ## Stage 8 — Assemble item scopes and YAML path/glob imports
 
 Scope: directive parsing and scoped acquisition/composition.