Add fixed destination path mapping

docs/internal/config.md

@@ -25,6 +25,7 @@ Defaults are applied in `ApplyDefaults`:
 - SSH backend `host_key_policy` defaults to `accept-new`;
 - destination publish policy defaults to source output only;
 - Markdown-to-HTML mode defaults to `sidecar` when a transform block is present and mode is omitted;
+- destination path mapping defaults to `preserve_relative`;
 - `transfer.on_destination_same` defaults to `skip`;
 - `transfer.on_destination_older` defaults to `replace`;
 - `transfer.on_destination_newer` defaults to `skip`;
@@ -32,12 +33,14 @@ Defaults are applied in `ApplyDefaults`:
 
 ## Validation responsibilities
 
-Validation requires at least one pipeline, slug-like unique pipeline ids, one source per pipeline, at least one destination, slug-like unique destination ids within each pipeline, backend-specific required fields, valid validation policy, valid publish and transform combinations, and valid transfer actions.
+Validation requires at least one pipeline, slug-like unique pipeline ids, one source per pipeline, at least one destination, slug-like unique destination ids within each pipeline, backend-specific required fields, valid validation policy, valid publish and transform combinations, valid destination path mapping mode, and valid transfer actions.
 
 Transfer validation accepts `replace` for `on_destination_newer` and `on_conflict`, but publish planning honors those destructive actions only when the current run explicitly requests force.
 
 `ValidatePublishTransformPolicy` is shared with publish planning so destination policy combinations are checked consistently. Publishing HTML requires an enabled Markdown-to-HTML transform in `sidecar` or `index` mode. Enabled Markdown-to-HTML config is rejected when `publish.html` is false. `input` is accepted only for enabled `index` mode. A publish policy must select source output, HTML output, or both.
 
+Destination path mapping accepts `preserve_relative` and `fixed`. The app layer applies the mapping when it selects destination bundle paths; config owns only YAML shape, defaulting, and validation.
+
 ## Executable support boundary
 
 Config validation accepts `local`, `ssh`, and `s3` backend shapes. Runtime execution opens all three through `internal/app`.

docs/internal/publish.md

@@ -6,7 +6,7 @@
 
 ## Inputs and outputs
 
-Inputs are a source bundle, source backend, destination backend, pipeline id, destination id, publish policy, transform policy, transformer resolver, transfer policy, destination bundle path, existing destination state, and whether explicit force was requested for the current run.
+Inputs are a source bundle, source backend, destination backend, pipeline id, destination id, publish policy, transform policy, transformer resolver, transfer policy, path mapping mode, destination bundle path, existing destination state, and whether explicit force was requested for the current run.
 
 Output is a plan with an action, reason, and selected source or generated outputs. Execution writes selected source files, generated files, and `.distributor.json` for publish or replacement actions.
 
@@ -24,7 +24,9 @@ Execution fails if a write, delete, state serialization, or context check fails.
 
 The current implementation publishes source files and Markdown-to-HTML outputs. Markdown sidecar mode writes same-directory `.html` outputs, and Markdown index mode writes `index.html`. Backend behavior is supplied through `internal/storage`; app runtime currently supplies local, SSH, and S3 backends.
 
-The package uses `internal/state` for destination comparison, `internal/storage` for IO, and the shared `internal/config` publish/transform policy helper for request validation. It resolves transforms through a narrow resolver supplied by the caller; concrete transform registration is owned by the app layer. It does not parse CLI flags or load config files.
+The package uses `internal/state` for destination comparison, `internal/storage` for IO, and the shared `internal/config` publish/transform policy helper for request validation. It resolves transforms through a narrow resolver supplied by the caller; concrete transform registration is owned by the app layer. It does not parse CLI flags, load config files, or choose which source bundles a destination receives.
+
+The app layer computes the destination bundle path before planning. `preserve_relative` destinations pass the source-root-relative bundle path. `fixed` destinations pass an empty destination bundle path, which means the destination backend root, and pass only the newest selected source bundle for that destination.
 
 ## Safety
 
@@ -37,6 +39,7 @@ Before changing publish behavior, inspect tests under `internal/publish` and run
 ## Invariants
 
 - Publish planning is deterministic for the same source, destination state, policies, and transform outputs.
+- Destination bundle paths are caller-supplied and are interpreted relative to the destination backend root.
 - Normal replacement deletes only managed paths recorded in existing state plus `.distributor.json`.
 - Forced replacement is explicit per run and deletes only within the destination bundle path.
 - Publish execution writes destination state after selected outputs are written.