docs: add roadmap with v0.2 plans and future considerations

Author: bug3

ROADMAP.md

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+# Roadmap
+
+## v0.2
+
+- [ ] **Inline template support** — Allow passing SQL strings directly without requiring a `.sql` file, reducing boilerplate for short one-off queries
+- [ ] **Template caching** — Cache loaded templates in a `Map` to avoid redundant `readFileSync` calls when `defineQuery` is used outside module scope
+- [ ] **TypeDescriptor `description` field** — Add an optional `description` property to `TypeDescriptor` so schema validation errors can display the expected format (e.g. `Expected: ISO date YYYY-MM-DD`)
+
+## Maybe
+
+- [ ] **Conditional blocks / IN-list expansion** — Support optional WHERE clauses (`{{#if}}`) and array expansion (`WHERE id IN ({{ids}})`). Would change the library's templating nature significantly — evaluate only if `.sql` file duplication becomes a real pain point
+- [ ] **Async `loadTemplate`** — Non-blocking file reads for server environments (Express, Fastify). Not needed for current Lambda/script use cases
+- [ ] **Query execution integration** — Return parameterized output or integrate with Athena SDK. Intentionally out of scope to preserve single responsibility — better suited as a separate package
+- [ ] **Parameterized output mode** — Return `{ sql: string, params: unknown[] }` with `$1, $2` placeholders for engines that support it. Not applicable to Athena but could broaden adoption
+- [ ] **Allowlist approach for `schema.string`** — Require a regex allowlist instead of relying on the denylist. Already achievable via custom schema types, so built-in support is low priority