Release PR for **spt-core v0.16.0** — the update-arc + CLI/UX release (W1–W6). Cut by deployah (publisher leg) off doyle-gated branch `v0.16.0-update-arc` @700b685.

## Cut mechanics (bump-in-PR)
- **Version bump 0.15.0 → 0.16.0** authored in this PR (`chore(release)`) so PR CI validates the tagged commit; tag on merge. Targeted first-party bump only (workspace.package.version + 11 first-party Cargo.lock entries — no third-party touched).
- **Release counter 35** at publish (cross-checked monotonic 34→35 against PUBLISHED `spt-x86_64-linux.release.json` on spt-releases).
- The `[0.16.0]` CHANGELOG section (doyle-vetted @700b685) becomes the GH Release notes verbatim — not re-scoped here.

## ⚠️ One extra commit beyond the bump — flag for doyle
`fix(cli): move REQ-DIGEST-CURSOR tag out of clap /// help text` (9e34161).

`xtask check` (docs-token gate) was **RED** on @700b685: the `[impl->REQ-DIGEST-CURSOR]` traceability tag sat inside the `///` doc-comments for `endpoint digest --last`/`--after`, leaking the internal req code into public `spt --help` and the published docs-site reference. CI would have redded the matrix. Fix moves the tag to an adjacent `//` line (same pattern the sibling `--json` field uses) — traceable coverage preserved, help no longer ships the code. 2-line change + regenerated `reference.md`.

## Local gates (post-bump, post-fix, this branch HEAD)
- `xtask gen` — no drift
- `xtask check` — OK (was RED pre-fix)
- `traceable-reqs check` — exit 0 (REQ-DIGEST-CURSOR still covered via the `//` tag)
- todlando's pre-bump matrix @700b685: 858/858 nextest · clippy 0 · traceable 306/0 · xtask no-drift

CI matrix fires both runners (kitsubito Linux + hfenduleam Windows). twohost rig skipped — zero cross-host networking seam in v0.16.0. doyle gates the matrix.