[ParallelEntrepreneur]

Part of #4. Split: this PR carries only the open-core half of the human-score work: the wire shape and the port contract. The stylometric detector, its measurements and the CLI opt-in are hosted-only. They moved to the private hosted repository (Ghostwritin/ghostwritin-cloud#2), which depends on this PR's head commit through an exact git pin. The branch was rebuilt on current main.

What changed (open)

• Score gains #[serde(default)] flags: Vec<Flag>.
  • A Flag is { start, end, reason }: byte offsets on char boundaries into the scored text. score_before indexes the request text and score_after indexes the rewrite.
  • Flags may overlap or nest and carry no words.
• FlagReason (#[non_exhaustive]): stock_phrase, uniform_rhythm.
• HumanScore port docs: an implementation stores and logs nothing, and its flags are offsets, never text.
• The wire-shape test covers flags and a score from before they existed.
• README: a short note on the score's shape under the API example.

The open API, Worker and CLI still ship NoHumanScore, so both scores are null, and the README status line is unchanged.

Moved to the private repository

• The crates/ghostwritin-detect crate (the Stylometric heuristic, the evaluate example and its tests).
• The CLI --human-score flag, now a ghostwritin-cloud binary there.
• The README "Human score" section with the measured HC3 and RAID error rates.

Verification (local; Actions is billing-blocked)

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• RUSTDOCFLAGS="-D warnings" cargo doc --no-deps --workspace
• cargo build --target wasm32-unknown-unknown -p ghostwritin-worker