feat(baton): discontinue title-scan; declared work_verb is the sole classification signal (#332)

[montfort]

Implements the architecture decision in #332, registered after the Sentinel adopter calibration (#331). The Baton-side prototype.

What

The declared work_verb (+ design_provenance), captured at authoring, becomes the sole authoritative classification signal. Title-scan is removed from the classification path entirely. A unit with no declared verb is unclassifiable → routed up to frontier conservatively, with a nudge to declare the verb. Never guessed.

• signals.rs — removed Cue/scan_cues/CUE_TABLE/matches_at_word_start; added WorkVerb (design/implement/audit/operate) + DesignProvenance (new/upstream), controlled-vocabulary.
• classify.rs — class: Option<TaskClass>; verb→class with implement + design_provenance=upstream → operator (residual-cognitive-load); undeclared → None. Declared = High confidence.
• route.rs — undeclared → frontier (conservative, R1).
• telemetry.rs — conflict_fraction → undeclared_fraction (the actionable nudge); saving now provably rests on declared units (low_confidence_savings_fraction → 0).
• units.rs — harvest work_verb/design_provenance from charter frontmatter + follow-up lines.

Why (adopter evidence)

The Sentinel calibration proved title-substring classification unsafe: high+medium precision 0.57 with 4 errors downward, all from object-vs-verb collisions (a module named Audit), incidental tokens (*.go filenames, commit hashes, make-target names). The title is unconstrained, schema-less, often-bilingual free text; reinforcing the scan per adopter is an unbounded keyword treadmill (the #321 anti-pattern). Fake savings are worse than an honest gap.

Verification

• cargo test -p straymark-baton ✓ — 78 tests (suite rewritten for the declared-verb model); clippy clean; no trace of scan_cues/Cue left.
• Sentinel dogfood (route --dry-run, read-only, git status intact): the legacy corpus declares no verbs → 100% undeclared → all frontier → 0 saving → "not routable" + "declare a work_verb" nudge. The truthful clean-cut behaviour — the gap is an action, not a fake number.

Scope boundary (deferred on purpose)

• Framework graduation — work_verb/design_provenance as first-class governance fields (schemas, templates, validator nudge) is a fw-X.Y.Z change, gated on schema ratification (the adopter asked not to instrument against an unratified schema).
• Finer-grained declaration (batch/task have no frontmatter slot), forward-validation (StrayMark's job, varied corpus), and B2 auto-assist for design_provenance are tracked under Decisión: descontinuar title-scan; work_verb declarado como única señal de clasificación #332.

Closes the prototype step of #332.

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