feat(iace): Risikograph EN ISO 13849-1 PLr + Methoden-Kopf im Bericht

Phase 17 of the risk-assessment polish. Two pieces:

A) PLr per EN ISO 13849-1 Anhang A (Risikograph)
   - HazardPattern.DefaultAvoidability (1 = P1, 2 = P2). Optional;
     defaults to P1 if unset (conservative — operator can raise after
     review).
   - ComputePLr(s,f,p) implements the canonical 8-leaf binary tree
     (S1F1P1 -> a, ..., S2F2P2 -> e). Pinned by 8 table-driven tests.
   - SeverityToS / ExposureToF map the existing 1-5 fields to the
     binary S/F at the documented threshold (3).
   - At project initialise, every hazard's Description is appended
     with "Risikograph EN ISO 13849-1 (Anhang A): S2 · F1 · P1 -> PLr c"
     so the audit value is visible without leaving the hazard view.
   - PatternMatch carries DefaultAvoidability so the init handler can
     pick it up without a second pattern lookup.

B) Methoden-Kopf am Bericht
   - GET /clarifications.html now opens with a standardised methodology
     block: ISO 12100 Anhang B (hazard ID) + ISO 13849-1 Anhang A
     (PLr graph) + ISO 12100 6.2/6.3/6.4 (reduction hierarchy). Same
     wording on every export, ready for the Anlagenbauer-Uebergabe.
   - Only norm identifiers — no norm text reproduced.

C) ISO12100Section in Hazard Description
   - When a pattern is labeled with ISO12100Section, the hazard
     description gets a "Klassifikation: EN ISO 12100 Anhang B,
     Abschnitt 6.3.5.4" suffix. Provenance for the auditor.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ai-compliance-sdk/internal/iace/pattern_engine.go

@@ -70,6 +70,7 @@ type PatternMatch struct {
 	SuggestedMeasureIDs      []string `json:"suggested_measure_ids,omitempty"`
 	ClarificationQuestionsDE []string `json:"clarification_questions_de,omitempty"`
 	ISO12100Section          string   `json:"iso_12100_section,omitempty"`
+	DefaultAvoidability      int      `json:"default_avoidability,omitempty"`
 }
 
 // HazardSuggestion is a suggested hazard from pattern matching.
@@ -226,6 +227,7 @@ func (e *PatternEngine) Match(input MatchInput) *MatchOutput {
 			SuggestedMeasureIDs:      p.SuggestedMeasureIDs,
 			ClarificationQuestionsDE: p.ClarificationQuestionsDE,
 			ISO12100Section:          p.ISO12100Section,
+			DefaultAvoidability:      p.DefaultAvoidability,
 		})
 
 		for _, cat := range p.GeneratedHazardCats {