docs(retrieval): add ARCHITECTURE.md and mohu-retrieval crate stub

[aaniya22]

What

Adds ARCHITECTURE.md documenting the retrieval design constraints, and an empty crates/mohu-retrieval stub to reserve the namespace. Addresses part of #286.

Why

mohu's design pillars (Rayon parallelism by default, Arrow-native memory layout, zero-copy Python interop) conflict with FAISS, the most widely-adopted ANN library — it's not Arrow-native and is single-threaded by default. Before any retrieval backend gets implemented, it's worth documenting that conflict and the options under discussion, and giving contributors a clear crate to build toward instead of an ambiguous "where does this go?" question.

How

• ARCHITECTURE.md: documents the pillar conflict, and the three options raised in architecture: FAISS chosen as sole vector index but is CPU-bound, single-threaded-by-default, and non-Arrow-native — conflicts with mohu's core "parallel by default, Arrow-native" design pillars #286 (Rust-native ANN library like usearch/hnswlib, a carefully-wrapped FAISS with an explicit copy boundary, or a native Rayon-parallel HNSW implementation as a mohu crate). No decision is made here — this is scoping, not a design doc for an implementation.
• crates/mohu-retrieval: empty placeholder crate, registered in the workspace, to reserve the namespace.

Important — scope note: #286 also asks to relocate a root-level vector_store.py into examples/faiss_integration/. I checked (git log --all -- "*vector_store.py*", plus a repo-wide grep for faiss) and this file doesn't exist anywhere in this repo's history — there's no FAISS code, and nothing to relocate. This PR covers only the documentation and crate-stub portions of the issue. Happy to close out the rest of #286 once there's clarity on whether that file was expected to exist already or is part of unstarted future work.

Checklist

• cargo test --workspace passes
• cargo clippy --workspace -- -D warnings passes
• cargo fmt --all applied
• CHANGELOG.md updated (if user-facing change) — not user-facing, no behavior change
• Benchmarks added or updated (if performance-sensitive path) — not applicable, no implementation

[github-actions]

PR Check Summary

Item	Value
Branch	docs/retrieval-architecture-286
Changed crates	crates/mohu-retrieval
Files changed	5

CI will run: build, test, clippy, fmt, cargo-deny, DCO, semver.
Reviewer assigned from CODEOWNERS.

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📥 Commits

Reviewing files that changed from the base of the PR and between 3c07775 and 3619533.

⛔ Files ignored due to path filters (1)

• Cargo.lock is excluded by !**/*.lock

📒 Files selected for processing (4)

• ARCHITECTURE.md
• Cargo.toml
• crates/mohu-retrieval/Cargo.toml
• crates/mohu-retrieval/src/lib.rs

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