docs(specifications): add helix-db-feature-ideas spec

[noahgift]

Summary

Adds docs/specifications/helix-db-feature-ideas.md — a cross-pollination spec evaluating which helix-db patterns are worth adopting in aprender. Nine candidate proposals (HELIX-IDEA-001..009), each scoped with effort, target crate, non-goals, open questions, and acceptance signals.

Recommended (high priority):

• HELIX-IDEA-001 — Persistent on-disk HNSW (extends in-memory aprender-core/src/index/hnsw.rs)
• HELIX-IDEA-002 — Inventory-based MCP handler auto-registration (replaces hardcoded Vec at aprender-mcp/src/server.rs:221-233)
• HELIX-IDEA-005 — Hybrid retrieval (BM25 + dense); the RAG spec lists this as a design goal but no implementation exists
• HELIX-IDEA-006 — Reranking pipeline (RRF / MMR / cross-encoder)
• HELIX-IDEA-007 — Snapshot / atomic backup primitive
• HELIX-IDEA-009 — Constant-time API-key auth for apr serve

Speculative / deferred:

• HELIX-IDEA-003 — Compile-time-validated DSL macro pattern (needs concrete pain point first)
• HELIX-IDEA-004 — Multi-target deployment scaffolding (deferred until hosted-inference direction is set)
• HELIX-IDEA-008 — Schema versioning macro (defer until first painful registry migration)

Explicitly rejected (with reasons in §3): LMDB storage swap, HelixQL the language, helix-db graph traversal model, embedding-provider abstraction, browser dashboard, vendor-specific metrics pipeline, heed3 for registry (already SQLite).

What this is and isn't

• This is an ideas / proposal document, not an implementation. No code changes.
• §1.3 grounds claims in verified facts about aprender's current state (HNSW LOC, registry storage, MCP wiring, redb availability).
• §6 contains a falsification log: the initial draft incorrectly claimed MCP handler discovery was contracts-mediated; corrected to "contracts mediate schema, manual hardcoded Vec mediates discovery." That correction sharpens HELIX-IDEA-002's value.

Test plan

• Markdown renders cleanly on GitHub
• All file paths cited in the spec resolve in the current main (e.g., crates/aprender-core/src/index/hnsw.rs, crates/aprender-mcp/src/server.rs:221-233)
• No code changes — ci / gate and workspace-test should pass without rebuild impact
• Reviewers should sanity-check effort estimates and challenge the "rejected" reasoning in §3

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