feat(sqlite): initial stable release v0.9.0

Pure-Go CGO-free SQLite client with launcher lifecycle, write-mutex serialisation, health check, unit-of-work via context injection, and structured error mapping.

What's included:
- Executor / Tx / Client / Component interfaces using database/sql native types
- Tx.Commit() / Tx.Rollback() without ctx, matching the honest database/sql contract
- New(logger, cfg) constructor; database opened in OnInit
- Config struct with env-tag support; default Pragmas: WAL + 5s busy timeout + FK enforcement
- PRAGMA foreign_keys = ON enforced explicitly in OnInit
- writeMu sync.Mutex acquired by UnitOfWork.Do to serialise writes and prevent SQLITE_BUSY
- UnitOfWork via context injection; GetExecutor(ctx) returns active Tx or *sql.DB
- HandleError mapping SQLite extended error codes to xerrors codes (unique/primary-key → AlreadyExists, foreign-key → InvalidInput, ErrNoRows → NotFound)
- health.Checkable at LevelCritical; pure-Go modernc.org/sqlite driver (CGO_ENABLED=0 compatible)

Tested-via: todo-api POC integration
Reviewed-against: docs/adr/

docs/adr/ADR-003-fk-enforcement-pragma.md

@@ -0,0 +1,54 @@
+# ADR-003: Foreign Key Enforcement via PRAGMA and DSN
+
+**Status:** Accepted
+**Date:** 2026-03-18
+
+## Context
+
+SQLite disables foreign key constraint enforcement by default for backwards compatibility.
+Applications that define `REFERENCES` clauses in their schema will silently insert orphaned
+rows unless they explicitly enable enforcement. This is a common source of data integrity bugs.
+
+## Decision
+
+Foreign key enforcement is enabled at two points:
+
+1. **DSN parameter** — The default `Pragmas` config value includes `_fk=true`:
+   ```
+   ?_journal=WAL&_timeout=5000&_fk=true
+   ```
+   This sets `PRAGMA foreign_keys = ON` for every connection opened via the DSN.
+
+2. **OnInit explicit PRAGMA** — After opening the database pool, `OnInit` executes an
+   additional `PRAGMA foreign_keys = ON` call:
+   ```go
+   if _, err := db.Exec("PRAGMA foreign_keys = ON"); err != nil {
+       _ = db.Close()
+       return fmt.Errorf("sqlite: enable foreign keys: %w", err)
+   }
+   ```
+   If this call fails, `OnInit` returns an error and the pool is closed, preventing startup
+   with an unsafe configuration.
+
+The redundancy is deliberate: the DSN parameter may be overridden by callers who supply a
+custom `Pragmas` value, but the `OnInit` PRAGMA call always runs and fails loudly if it cannot
+enforce foreign keys.
+
+`HandleError` maps `SQLITE_CONSTRAINT_FOREIGNKEY` (error code 787) to
+`xerrors.ErrInvalidInput` so that foreign key violations surface as validation errors to
+callers rather than opaque internal errors.
+
+## Consequences
+
+**Positive:**
+- Foreign key constraints are always active when using the default configuration.
+- Failure to enable them is a startup error, not a silent misconfiguration.
+- Violations produce a structured `xerrors.ErrInvalidInput` error.
+
+**Negative:**
+- Callers who deliberately omit `_fk=true` from a custom `Pragmas` string still get the
+  enforcement applied by the `OnInit` PRAGMA. There is no opt-out without modifying the source.
+- `PRAGMA foreign_keys = ON` must be set per-connection; `database/sql` connection pooling
+  means this implicit approach (via DSN) can behave differently under pool pressure. The
+  explicit `OnInit` PRAGMA mitigates this for the initial connection but cannot guarantee it
+  for all pooled connections when `MaxOpenConns > 1`.