docs: add public ROADMAP (#165)

Add a public ROADMAP.md describing project direction and pointing to the issue tracker as the authoritative view. Closes the OpenSSF Silver documentation_roadmap gap.

ROADMAP.md

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+# Roadmap
+
+This roadmap describes the project's direction at a high level. It is
+intentionally non-binding: priorities shift with feedback and available time,
+and the authoritative, up-to-date view of planned work is the
+[issue tracker](https://github.com/wickra-lib/wickra/issues). Shipped changes
+are recorded in [`CHANGELOG.md`](CHANGELOG.md).
+
+## Status
+
+Wickra is **pre-1.0**. The public API is largely stable but may still change in
+minor releases; breaking changes are called out in the changelog.
+
+## Themes
+
+- **Indicator coverage.** Continue broadening the indicator catalogue across
+  families (trend, momentum, volatility, volume, statistics, market profile,
+  and more), each with the same streaming/batch parity and test guarantees.
+- **API stabilization toward 1.0.** Settle the public `Indicator` and
+  `BarBuilder` traits and the binding surfaces, then commit to semantic
+  versioning stability for a 1.0 release.
+- **Performance.** Keep per-tick updates O(1) and maintain the benchmark suite;
+  investigate further allocation and cache improvements.
+- **Bindings parity.** Keep the Python, Node.js and WebAssembly bindings in
+  lockstep with the Rust core, including type stubs and platform coverage.
+- **Documentation.** Maintain a deep-dive page per indicator on
+  <https://docs.wickra.org>, plus quickstarts and cookbook material.
+- **Project health.** Maintain test coverage, static and dynamic analysis,
+  signed releases, and supply-chain monitoring.
+
+## How to influence the roadmap
+
+Open or comment on an issue, or start with the
+[feature-request template](.github/ISSUE_TEMPLATE/feature_request.md).
+Well-scoped proposals and pull requests are the most effective way to move an
+item forward.