feat(config): add frDate helper for French natural-language dates

Go date layouts only know English month names, so date(value, layout)
cannot parse "le 19 janvier 2026". frDate(text) extracts and parses such
dates (day optional, abbreviated months, accents optional) into a native
date; no match is a rule error routing the document to pending/.

Documented in arch.md §5, covered by unit tests.

archivist/internal/config/config.go

@@ -190,6 +190,7 @@ func exprOptions() []expr.Option {
 	return []expr.Option{
 		expr.Function("firstMatch", FirstMatch),
 		expr.Function("period", Period),
+		expr.Function("frDate", FrenchDate),
 	}
 }
 
@@ -396,6 +397,78 @@ func toString(v any) string {
 	}
 }
 
+// frDateRe matches a French natural-language date, with or without a day:
+// "le 19 janvier 2026", "Janvier 2026", "1er févr. 2026". Abbreviated month
+// names are accepted, accents are optional.
+var frDateRe = regexp.MustCompile(`(?i)(?:le\s+)?(?:(1er|\d{1,2})\s+)?(janv\.?|janvier|f[eé]vr\.?|f[eé]vrier|mars|avr\.?|avril|mai|juin|juil\.?|juillet|ao[uû]t|sept\.?|septembre|oct\.?|octobre|nov\.?|novembre|d[eé]c\.?|d[eé]cembre)\s+(\d{4})`)
+
+// frMonths maps the folded month names to their numbers. Keys are lowercase
+// and unaccented; foldFrench normalizes what the regex captured.
+var frMonths = map[string]time.Month{
+	"janv": time.January, "janvier": time.January,
+	"fevr": time.February, "fevrier": time.February,
+	"mars": time.March,
+	"avr": time.April, "avril": time.April,
+	"mai": time.May,
+	"juin": time.June,
+	"juil": time.July, "juillet": time.July,
+	"aout": time.August,
+	"sept": time.September, "septembre": time.September,
+	"oct": time.October, "octobre": time.October,
+	"nov": time.November, "novembre": time.November,
+	"dec": time.December, "decembre": time.December,
+}
+
+// foldFrench lowercases and strips the French diacritics used in month
+// names plus the abbreviation dot, so the lookup tolerates "Février",
+// "FEVRIER" and "févr." alike.
+func foldFrench(s string) string {
+	r := strings.ToLower(s)
+	for _, a := range [][2]string{{"é", "e"}, {"è", "e"}, {"ê", "e"}, {"û", "u"}, {"â", "a"}} {
+		r = strings.ReplaceAll(r, a[0], a[1])
+	}
+	return strings.TrimSuffix(r, ".")
+}
+
+// FrenchDate is the frDate(text) helper (arch.md §5). It extracts and
+// parses a French natural-language date — "le 19 janvier 2026", the day
+// being optional ("Janvier 2026" gives the first of the month) and
+// abbreviated month names accepted ("1er févr. 2026") — into a native
+// date. Go layouts only know English month names, so date(value, layout)
+// cannot parse these. A text containing no French date is a rule error
+// and routes the document to pending/.
+func FrenchDate(args ...any) (any, error) {
+	if len(args) == 0 {
+		return nil, fmt.Errorf("frDate: text required")
+	}
+	text, _ := args[0].(string)
+	m := frDateRe.FindStringSubmatch(text)
+	if m == nil {
+		return nil, fmt.Errorf("frDate: no French date found")
+	}
+	day := 1
+	if m[1] != "" && !strings.EqualFold(m[1], "1er") {
+		d, err := strconv.Atoi(m[1])
+		if err != nil {
+			return nil, fmt.Errorf("frDate: %w", err)
+		}
+		day = d
+	}
+	month, ok := frMonths[foldFrench(m[2])]
+	if !ok {
+		return nil, fmt.Errorf("frDate: unknown month %q", m[2])
+	}
+	year, err := strconv.Atoi(m[3])
+	if err != nil {
+		return nil, fmt.Errorf("frDate: %w", err)
+	}
+	d := time.Date(year, month, day, 0, 0, 0, 0, time.UTC)
+	if d.Month() != month || d.Day() != day {
+		return nil, fmt.Errorf("frDate: %d %s %d is not a valid date", day, m[2], year)
+	}
+	return d, nil
+}
+
 var frenchMonths = []string{"Janvier", "Février", "Mars", "Avril", "Mai", "Juin",
 	"Juillet", "Août", "Septembre", "Octobre", "Novembre", "Décembre"}