feat: add PDF dossier generation (internal/dossier)

Content assembly (BuildContent) is separate from PDF drawing (Render),
so the actual business logic — what goes into the evidence dossier, in
what form, with which mandatory fields — is unit-testable without
parsing PDF bytes. BuildContent refuses to produce a dossier missing
its evidentiary fields (timestamp token, asset/metadata hash, platform)
rather than emitting one with silently empty proof sections. Every
dossier carries the "this is not legal advice" disclaimer required by
CLAUDE.md's guardrails.

Uses github.com/go-pdf/fpdf (actively maintained fork of jung-kurt/
gofpdf, no dependencies beyond the Go stdlib) for rendering. Its core
fonts use cp1252 internally, so a small cp1252.map (copied from the
fpdf module, embedded via go:embed) drives UnicodeTranslator — German
umlauts render correctly without needing an external font file at
runtime, keeping Deklarix a single binary. Verified visually with
pdftotext/pdfinfo against a generated sample.

Tests build a real, structurally valid RFC-3161 token offline (a
throwaway self-signed cert + timestamp.Timestamp.CreateResponse), so
BuildContent/Render/Generate are fully tested without hitting a real
TSA — unlike the network-gated integration test in internal/evidence.

Also adds evidence.TimestampTime(), extracted from the parsing logic
already used by the TSA client, since the dossier needs to show the
timestamped time to a human reader.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/evidence/parse.go

@@ -0,0 +1,18 @@
+package evidence
+
+import (
+	"fmt"
+	"time"
+
+	"github.com/digitorus/timestamp"
+)
+
+// TimestampTime liefert den im RFC-3161-Token bescheinigten Zeitpunkt,
+// z. B. für die Anzeige im Nachweis-Dossier.
+func TimestampTime(token []byte) (time.Time, error) {
+	ts, err := timestamp.Parse(token)
+	if err != nil {
+		return time.Time{}, fmt.Errorf("evidence: timestamp token parse: %w", err)
+	}
+	return ts.Time, nil
+}