refactor: migrate repository to Cargo workspace (#20)

[meymchen]

Closes #20.

Converts the single lspf crate into a Cargo workspace, the packaging foundation for the installable lspf-hello template server (PRD #19).

Changes

• Root Cargo.toml is now a workspace manifest (members = ["crates/lspf", "crates/lspf-hello"], resolver = "3"), with shared metadata in [workspace.package] and common deps in [workspace.dependencies].
• crates/lspf/ — framework source moved unchanged (git mv, history preserved). Still a single crate per ADR 0013.
• crates/lspf-hello/ — the old examples/hello promoted to an installable binary crate with its own Cargo.toml and a default lspf-hello binary target. Depends on lspf via both path and version so local development uses the latest framework code while crates.io publishing stays possible.
• Tests — framework-level tests stay in crates/lspf/tests/; the end-to-end smoke test moves to crates/lspf-hello/tests/ and drives target/debug/lspf-hello through Cargo's CARGO_BIN_EXE_lspf-hello (replacing cargo build --example hello).

Approach

Built test-first (TDD): confirmed cargo build -p lspf-hello failed (RED), performed the migration, then drove the relocated smoke suite against the new binary to GREEN.

Acceptance criteria (#20)

• Root Cargo.toml becomes a workspace manifest.
• Framework source moves to crates/lspf/ as a workspace member.
• examples/hello moves to crates/lspf-hello/ with its own Cargo.toml and default binary target.
• lspf-hello depends on lspf via both path and version.
• cargo test --workspace, cargo clippy --workspace --all-targets, and cargo fmt --all pass.
• ADR 0013 respected: lspf stays a single crate with feature flags; only the example/template binary is split out.

Scope notes

Relocating the smoke test was pulled in here because removing the example breaks the old cargo build --example hello path, and a green cargo test --workspace is a #20 acceptance criterion. The remaining slices are left to their own issues:

• vscode-test-client binary path → [VS Code/Zed MVP] Migrate smoke tests and VS Code test client to lspf-hello #21
• VS Code / Zed docs + README example references → [VS Code/Zed MVP] Add VS Code and Zed integration documentation #22
• CI workflow → [VS Code/Zed MVP] Update CI workflow for workspace #23 (current cargo test --all-features --all-targets already runs at the workspace root and stays green)

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