refactor: establish ecstore layout foundation (#3645)

docs/architecture/ecstore-layout-boundary.md

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+# ECStore Layout Boundary
+
+This document records the `E-001` and `E-SET-001` foundation slice for the
+architecture migration.
+
+## Directory Skeleton
+
+The ECStore migration uses these internal ownership buckets before any pure
+file moves:
+
+- `api`: facade and compatibility re-export ownership.
+- `core`: store facade, object, bucket, list, multipart, and heal paths.
+- `layout`: static endpoint, disk, pool, and set layout descriptions.
+- `disk`: local disk, format, health, and disk error ownership.
+- `erasure`: erasure coding and bitrot ownership.
+- `metadata`: bucket metadata, config object-store, and data-usage ownership.
+- `cluster`: remote disk, peer, lock, membership, and health-control-plane
+  ownership.
+- `services`: lifecycle, replication, tier, notification, rebalance, and
+  metrics service ownership.
+
+## Static Set Layout
+
+Static layout is derived from persisted `FormatV3` data and input endpoint
+expansion. It may describe:
+
+- deployment id;
+- set count and drives per set;
+- disk UUID positions inside `format.erasure.sets`;
+- distribution algorithm;
+- endpoint grouping produced before runtime disk initialization.
+
+Static layout must not own disk handles, lock clients, reconnect loops, repair
+state, or shutdown signaling.
+
+## Runtime Set Orchestration
+
+Runtime orchestration remains owned by `Sets` and `SetDisks` until a later pure
+move. It may describe:
+
+- flat disk index to `(set_index, disk_index)` mapping;
+- per-set local disk replacement after distributed setup detection;
+- per-set lock-client host deduplication;
+- endpoint reconnect monitoring and runtime shutdown signaling;
+- read/write/heal/list orchestration over initialized disks.
+
+## Preservation Rules
+
+- Object-to-set hashing and distribution algorithm selection must not change.
+- Format `sets` ordering and disk UUID position lookup must not change.
+- Local disk replacement and lock-client mapping stay runtime-only.
+- Later file moves must keep old public paths or add explicit compatibility
+  coverage before deleting them.