Clarify unresolved source destination evidence in implementation plan

docs/roadmap/implementation.md

@@ -50,6 +50,9 @@ type SequenceComposition struct {
 
 - Add `SequenceCompositions []SequenceComposition` to `Definition[T]`.
 - Add `PlacementMode PlacementMode` to `Input` and `SourceInfo`.
+- Add `DestinationUnresolved bool` to `SourceInfo`. False preserves the existing
+  exact-destination interpretation; true explicitly marks the enclosing-container
+  fallback described below. This is output evidence, not an input option.
 - Add both `Destination Path` and `PlacementMode PlacementMode` to `Options[T]`.
   Ordinary `Load` needs the same per-source controls as the layered entry point;
   its overlay and CLI coordinates remain application-rooted.
@@ -131,6 +134,37 @@ inventing an all-errors guarantee or freezing incidental message wording.
    Output evidence may contain such indexes even though input destinations and
    registrations accept only mapping-key segments.
 
+### Unresolved destination evidence
+
+`SourceInfo.DestinationUnresolved` describes whether that snapshot's `Destination`
+is the source's exact effective destination container:
+
+- When false, `Destination` is exact. This says nothing about whether the load
+  succeeded or the source's values survived later replacement. An item source
+  can have an exact sequence-container destination before its own item index is
+  assigned, because that index is not part of its reported container path.
+- When true, an enclosing item index needed for the destination has not been
+  assigned. `Destination` is only the nearest resolved enclosing collection
+  container, not the source's actual mapping destination or nested sequence
+  destination. Do not append the unresolved local suffix as though the missing
+  index were a mapping key, or use a fabricated index.
+- For example, a mapping helper destined for `prompts[1].metadata` reports the
+  container `prompts` with `DestinationUnresolved: true` until the outer item
+  index is assigned. Later snapshots report `prompts[1].metadata` with false.
+  The item-root source itself reports `prompts` with false when that container
+  is known. The placement mode continues to describe each source's own mode.
+- Apply this distinction to every source-record exposure, including source
+  records available during callbacks and failed-load records. Once projection
+  resolves the container, subsequent snapshots report the exact path with false.
+  Previously returned detached snapshots retain their original path and flag.
+  Completed successful loads report resolved containers; early failures may
+  contain both resolved and unresolved source records.
+
+This flag makes the existing planned fallback observable without adding a public
+item-index API, promising permanent indexes, or reconstructing unknown placement
+after a failure. Physical source locations, source IDs, and source-local version
+evidence retain their existing meaning regardless of the flag.
+
 Choose private helper names and file splits to fit existing responsibilities.
 Do not introduce a provider framework, generic merge callbacks, a second loading
 lifecycle, or a public identity-selector API to implement these mechanics.
@@ -307,6 +341,8 @@ Scope: placement primitives and engine per-input options; not public exposure.
 - Add the private placement mode to engine input/request/options/source records,
   with preflight validation. Keep mapping placement unchanged. A valid item mode
   needs a nonempty mapping-key destination relative to its current mapping scope.
+- Carry the destination-resolution state in engine source records so later
+  scoped acquisition can distinguish exact containers from unresolved fallbacks.
 - Extend placement to wrap one completed source mapping in a singleton sequence
   at that destination. Preserve mapping-root input validation; do not accept raw
   source sequence/scalar roots. A nil payload stays absent, and `{}` contributes
@@ -342,6 +378,10 @@ Scope: directive parsing and scoped acquisition/composition.
   composer. Assign outer ranges there, then project the retained item body and
   helper events through those ranges. Do not emit each ordinary helper as a
   separate record or validate its identity independently.
+- Maintain source destination-resolution state as enclosing ranges are assigned.
+  Before assignment, source snapshots use the explicitly marked fallback from
+  the fixed design; after assignment, project the exact container. Preserve
+  already-returned snapshots rather than updating them in place.
 - Cover path and glob item imports, a helper-provided ID, relative and transitive
   helper destinations, mixed authored lists/items, optional unmatched imports,
   empty items, nested placement, default-mode independence, and physical paths
@@ -424,19 +464,25 @@ Scope: root facade declarations, conversions, and public integration tests.
   describing defaults, option failures, and ownership. Convert Definition
   registrations and paths, ordinary Options destinations/modes, layered Inputs,
   and successful/failed SourceInfo records in both directions as applicable.
+  Convert `DestinationUnresolved` alongside `Destination` and `PlacementMode`.
   Update callback-entry source conversions too, not only `Result.Sources`.
 - Preserve immutable source snapshots, reader single-use behavior, callback
   lifetime, independent loads, and no public exposure of engine/parser types.
   SourceInfo reports its own selected mode; CLI/overlay sources use mapping mode
   unless an actual supported item placement was applied.
 - For a completed load, resolve nested source containers through their owning
-  item projection. On an early failure retain acquired evidence without inventing
+  item projection. Apply the fixed unresolved-destination contract to callbacks
-  an index that was never assigned: keep an unresolved nested source's Destination
+  and early failures: a fallback container always has `DestinationUnresolved`
-  at its nearest resolved enclosing collection container, rather than flattening
+  set to true; an exact container has false. Failed-load records describe
-  away an index and reporting a fictitious effective mapping path. Once assigned,
+  acquisition context, not a usable partial configuration. Do not add a public
-  use the projected destination. Failed-load records describe acquisition context,
+  index or identity-address API or promise resolution of every failed source's
-  not a usable partial configuration. Do not add a public index or identity-address
+  final placement.
-  API or promise resolution of every failed source's final placement.
+- Test an unresolved helper record exposed during a callback and on early
+  failure, its exact projected destination after assignment, and a failure after
+  assignment that retains the exact destination. Distinguish an exact item-root
+  container from a helper fallback with the same path. Verify nested-item
+  fallback, preserved snapshot ownership, and false for unchanged mapping-only
+  sources. Reuse fixtures where these scenarios share the same lifecycle path.
 - Public tests demonstrate ordinary and layered opt-in, an item-import glob,
   authored-list/item interoperability, one-source validation, helper origins and
   resource contexts at shifted indexes, both option-error entry points, detached
@@ -444,9 +490,12 @@ Scope: root facade declarations, conversions, and public integration tests.
   edge cases in engine tests.
 - Update `docs/config.md` for the now-public registration controls, ordinary
   destination options, complete placement/evidence behavior, and strict-versus-
-  replacement validation boundary. Update the architecture policy's composition
+  replacement validation boundary. Document `DestinationUnresolved` and its
-  summary to recognize the registered-sequence rule and link ADR 0014. Stage 13
+  fallback meaning in both `SourceInfo` comments and the owning configuration
-  reviews and completes the surrounding guidance, not these canonical contracts.
+  contract, including callback and failure snapshots. Update the architecture
+  policy's composition summary to recognize the registered-sequence rule and
+  link ADR 0014. Stage 13 reviews and completes the surrounding guidance, not
+  these canonical contracts.
 
 Validation: focused facade tests,
 `go test . -run 'TestPublicFacadeBoundary|TestDependencyImportBoundary'`,