# PR-35 Packing-Aware Walkers and Runtime Materialization Preparation

Domain Owner: `docs/packer`
Cross-Domain Impact: `docs/studio`

## Briefing

Pack execution now depends on a frozen in-memory execution snapshot, but the repository deliberately chose not to fork discovery into one runtime walker stack and one packing walker stack.

This PR prepares the existing walker/materialization path so it can serve both runtime reads and pack execution with explicit policy.

## Objective

Prepare walkers and runtime materialization for packing-aware projection without implementing final pack emission yet.

## Dependencies

- [`../decisions/Pack Wizard Pack Execution Semantics Decision.md`](../decisions/Pack%20Wizard%20Pack%20Execution%20Semantics%20Decision.md)
- [`../decisions/Tile Bank Packing Materialization Decision.md`](../decisions/Tile%20Bank%20Packing%20Materialization%20Decision.md)
- `PR-34` for diagnostics already present in the walk/materialization path

## Scope

- add optional content bytes to runtime walk projection
- introduce or finish the accepted materialization config/policy shape
- keep family walkers discovery-oriented
- enable packing-focused projection filtering over discovered probes
- make build-relevant content bytes available for packing snapshots only

## Non-Goals

- no final `packWorkspace(...)` implementation yet
- no staging/promotion yet
- no tile-bank payload bytes emitted yet
- no alternative walker stack

## Method

1. Add `Optional<byte[]>` support to `PackerRuntimeWalkFile`.
2. Ensure full probes can still carry bytes internally during discovery.
3. Materialize runtime projection using explicit config/policy:
   - runtime mode suppresses optional bytes
   - packing mode injects optional bytes
   - projection filter keeps only build-relevant selected probes
4. Keep family walkers focused on relevance-by-family.
5. Add tests showing:
   - runtime default projection does not retain bytes
   - packing projection retains bytes only for relevant selected files
   - the same discovery path serves both modes

## Acceptance Criteria

- the repository has one walker discovery path, not two divergent ones
- packing projection can freeze build-relevant bytes in memory
- runtime-backed reads do not become wasteful full-memory mirrors
- selected build inputs survive into runtime walk projection with optional bytes when packing mode is requested

## Validation

- targeted runtime materialization tests
- walker reuse regression tests
- cache integration review where relevant

## Affected Artifacts

- `prometeu-packer-v1` walker models
- `prometeu-packer-v1` runtime materializer/export path
- related tests under `prometeu-packer-v1/src/test`