Minimal workspace + crate layout

[sij411]

Description

Set up the minimal Rust workspace/crate layout needed to start Phase 1 core work.

Proposal

• Create a Cargo workspace
• Add crates/feder-vocab
• Add crates/feder-core
• Wire up mise tasks for check/fmt

Done when

• cargo check works locally
• mise run check (or equivalent) passes
• cargo fmt is covered via mise
• No runtime crates yet

Out of scope

• GitHub Actions / release automation
• Linux runtime / ESP32 runtime

[dahlia]

Quick question on feder-model: is this meant to cover what @fedify/vocab does on the TypeScript side: type-safe representations of Activity Vocabulary objects like Note, Create, Person, and so on?

If so, I think feder-vocab may be clearer. The W3C spec calls this the Activity Vocabulary, and the name would line up with @fedify/vocab. feder-model sounds broader to me, and I wouldn't immediately read it as Activity Vocabulary types.

If feder-model is meant to cover more than vocabulary types, that would change my read.

[sij411]

Agree to feder-vocab. Thanks!