[Schema] Extend Span.kind with 'formula'/'refrain' + Span.parallels for cross-sutta phrase recurrence

[anantham]

Context

Per phase-a curation log §10.3. Surfaced 2026-05-11.

The opening formula `evaṁ me sutaṁ` ("Thus have I heard") is the canonical opener across the Pāli canon — a kind of refrain at canon scale, not within-packet. The current `refrainId` field on `PaliWord` is within-packet only (mostly used for phrase repeats like the locative-Buddha-dwelling formula).

Proposed shape

type Span = {
  id: string;
  kind: 'quoted_speech' | 'cited_phrase' | 'parenthetical' | 'formula' | 'refrain';
  startWordId: string;
  endWordId: string;
  parallels?: ParallelRef[];   // for cross-sutta formula recurrence
  note?: string;
};

Use cases

• `evaṁ me sutaṁ` formula → marks all suttas that open this way
• The "Buddha was dwelling at X" frame opener → recurs at every sutta-opening
• Specific stock phrases for sermons, refrains, etc.

Affected code

• `types/suttaStudio.ts` — `Span` type
• Renderer: would need a visual treatment for canon-scale refrains (subtle, similar to existing within-packet `refrainId` styling)

Acceptance

• Schema field added (additive, non-breaking)
• One MN10 phase wires it through (likely phase-a, the opener formula)

Hit count

1/4 phases (phase-a; phase-b's "Buddha was dwelling" frame is the next candidate)