Add multi-database data layer

Introduce aether-data-schema and driver-specific schema generation for Postgres, MySQL, and SQLite.

Split data backends, lifecycle, repositories, and gateway runtime integration across database drivers.

Verified with cargo fmt --all --check, cargo clippy --workspace --all-targets -- -D warnings, and cargo test --workspace.

crates/aether-data/schema/README.md

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+# Aether Schema Source
+
+This directory is the schema maintenance workspace. The executable migrations
+remain in `../migrations/{postgres,mysql,sqlite}` because runtime code and
+existing deployments still reference those paths. The Postgres bootstrap
+snapshot is compiled from the source fragments here during `aether-data`
+builds, so there is no checked-in bootstrap artifact.
+
+The maintenance flow is:
+
+```bash
+bash crates/aether-data/schema/compose_schema.sh generate
+bash crates/aether-data/schema/compose_schema.sh compose
+bash crates/aether-data/schema/compose_schema.sh check
+```
+
+- `generate` renders `logical/*.toml` through `aether-data-schema` into
+  `generated/{postgres,mysql,sqlite}`. This is a build output, not another SQL
+  source to maintain.
+- `compose` rewrites the executable SQL from the manifest order.
+- `check` verifies generated output is current, confirms the bootstrap source
+  fragments still compose cleanly, and diffs each executable migration manifest
+  against the checked-in SQL.
+- `split` regenerates fragments from the executable SQL and is mostly for
+  rebaselining after a deliberate bulk rewrite.
+
+## What To Edit
+
+The schema workspace has three normal source areas:
+
+| Path | Role | Edit policy |
+|---|---|---|
+| `logical/*.toml` | Long-term logical table model shared by all SQL drivers. | Edit first for portable table-shape changes. |
+| `drivers/{postgres,mysql,sqlite}/` | Current maintenance fragments for executable SQL. | Edit only for deployment compatibility, ordering, or generator gaps. |
+| `bootstrap/postgres/` | Source fragments for the Postgres empty-database bootstrap snapshot. | Edit here when the bootstrap snapshot changes, then rebuild `aether-data` so `build.rs` regenerates the embedded snapshot. |
+
+Everything else is output:
+
+| Path | Role | Edit policy |
+|---|---|---|
+| `generated/{postgres,mysql,sqlite}/` | Machine-written SQL emitted from `logical/*.toml` for audit and drift detection. | Do not edit; regenerate with `compose_schema.sh generate`. |
+| `../migrations/` | Runtime SQL artifacts embedded or executed by the application. | Regenerate through `compose_schema.sh compose`; do not edit independently. |
+
+`generated/**` is deliberately checked in so reviews and CI can see exactly
+what the logical schema compiler emits for each driver. It is not a fourth SQL
+source of truth, and runtime code never loads migrations from it.
+
+`overrides/` is an exception bucket, not a regular source tree. Keep it empty
+except for its README until a real driver-specific SQL file is needed and added
+to a manifest.
+
+The generator can also be called directly:
+
+```bash
+cargo run -p aether-data-schema --bin aether-schema -- check
+cargo run -p aether-data-schema --bin aether-schema -- generate
+cargo run -p aether-data-schema --bin aether-schema -- print --driver postgres
+```
+
+## Logical Schema
+
+`logical/*.toml` is the long-term source for table definitions. It covers the
+clean baseline table set and the portable MySQL/SQLite table-creation
+migrations. The generator emits driver-specific SQL under `generated/`; those
+files include a directory README plus `Do not edit` headers and should only
+change through `compose_schema.sh generate`.
+
+`compose_schema.sh check` enforces two things:
+
+- generated SQL must match the current logical TOML source
+- bootstrap source fragments must still compose cleanly for the runtime build
+- required executable SQL tables must have logical definitions, so new portable
+  tables cannot bypass the single-maintenance-source path
+
+The migration path is incremental:
+
+1. Add a table/domain to `logical/*.toml`.
+2. Run `compose_schema.sh generate`.
+3. Compare generated SQL to the current driver fragments.
+4. Promote generated output into driver fragments only when that domain is
+   intentionally ready to stop being handwritten.
+5. Keep driver-specific special cases in explicit override fragments under
+   `overrides/` only when they cannot live cleanly in a driver fragment.
+6. Once a domain matches, move its baseline maintenance to generated output.
+
+The existing `drivers/postgres`, `drivers/mysql`, and `drivers/sqlite`
+fragment trees remain authoritative for executable migrations until a generated
+fragment is deliberately promoted.
+
+`overrides/` is reserved for rare driver-specific SQL that cannot be represented
+by logical schema or the normal driver fragments. Keep it small and explicit.
+
+## Targets
+
+| Target | Executable SQL | Source manifest |
+|---|---|---|
+| Postgres baseline | `migrations/postgres/20260403000000_baseline.sql` | `drivers/postgres/baseline/manifest.txt` |
+| Postgres empty-database snapshot | `aether-data` build output (`OUT_DIR/empty_database_snapshot.sql`) | `bootstrap/postgres/manifest.txt` |
+| MySQL baseline | `migrations/mysql/20260403000000_baseline.sql` | `drivers/mysql/baseline/manifest.txt` |
+| SQLite baseline | `migrations/sqlite/20260403000000_baseline.sql` | `drivers/sqlite/baseline/manifest.txt` |
+
+Driver baseline source manifests are kept as a small set of numbered SQL
+fragments. Postgres uses execution-phase fragments so the pg_dump ordering
+remains byte-for-byte stable when composed:
+
+- `001_types_and_tables.sql`
+- `002_defaults.sql`
+- `003_constraints.sql`
+- `004_indexes.sql`
+- `005_foreign_keys.sql`
+- `006_footer.sql`
+- `100_*` extension files for empty-database snapshot-only additions
+
+MySQL and SQLite use similarly numbered domain fragments (`001_identity.sql`
+through `006_usage.sql`) because their baselines are shorter and already
+organized by domain.
+
+The Rust migration tests compose these manifests too, so fragment drift is
+caught during `cargo test -p aether-data split_baseline_sources_match_executable_migrations`.