localise chrome, smooth prose, in English and Bengali

The engine now owns the words it puts on a page that the author did not write
(ADR-0034). `internal/render/chrome.go` holds the phrase table, Gregorian month
names and decimal digits per language, keyed phrase-then-language so both forms
sit side by side and a half-translated row is visible while reading. Templates
reach it through `t`, `num` and `day`, registered before parsing so a site
override's blocks may call them too.

The reference theme stops hardcoding English: "Newer", "Page 2 of 2" and every
date now come from the table, while `datetime` attributes stay ASCII because a
parser reads them.

Authored text gets goldmark's typographer and nothing else — quotes, dashes and
ellipses smoothed, code spans untouched because it works on the parsed tree. It
is a parser option rather than a function over a page, so the transforms counter
does not move; state.md now says why, so the next reader does not miscount.

Deliberately absent: relative dates, which need a validity window that only the
cache entry will have, and body widow prevention, which cannot be done safely by
a pass over rendered HTML.

internal/render/chrome.go

@@ -0,0 +1,105 @@
+package render
+
+import (
+	"fmt"
+	"html/template"
+	"strconv"
+	"strings"
+	"time"
+
+	"khosra/internal/content"
+)
+
+// Chrome is every word the engine puts on a page that the author did not write, in each language it
+// knows (ADR-0034). Keyed by phrase then language, so both forms sit side by side and a missing one is
+// visible while reading rather than at request time.
+//
+// A phrase may carry %s placeholders, filled in order by the caller. Templates reach these through the
+// `t`, `num` and `day` functions; nothing here is content, and content never comes from here.
+var chrome = map[string]map[string]string{
+	"newer":    {"en": "Newer", "bn": "নতুন"},
+	"older":    {"en": "Older", "bn": "পুরোনো"},
+	"empty":    {"en": "Nothing here yet.", "bn": "এখনও কিছু নেই।"},
+	"page-of":  {"en": "Page %s of %s", "bn": "পৃষ্ঠা %s / %s"},
+	"position": {"en": "%s of %s", "bn": "%s / %s"},
+}
+
+// months are Gregorian month names per language, indexed by [time.Month]-1.
+var months = map[string][]string{
+	"en": {"January", "February", "March", "April", "May", "June",
+		"July", "August", "September", "October", "November", "December"},
+	"bn": {"জানুয়ারি", "ফেব্রুয়ারি", "মার্চ", "এপ্রিল", "মে", "জুন",
+		"জুলাই", "আগস্ট", "সেপ্টেম্বর", "অক্টোবর", "নভেম্বর", "ডিসেম্বর"},
+}
+
+// digits are the decimal digits of a script, indexed 0-9. A language absent here uses ASCII.
+var digits = map[string][]rune{
+	"bn": []rune("০১২৩৪৫৬৭৮৯"),
+}
+
+// funcs are the chrome helpers a template may call. Registered before parsing, so a site root's override
+// blocks may use them too.
+var funcs = template.FuncMap{"t": text, "num": numerals, "day": day}
+
+// text is one chrome phrase in a language, with any %s placeholders filled.
+//
+// It cannot fail: an unknown language falls back to the default locale and an unknown phrase returns its
+// own key, because a missing translation must never blank a page or break a render (invariant 1).
+func text(lang, key string, args ...string) string {
+	forms := chrome[key]
+	phrase, ok := forms[lang]
+	if !ok {
+		phrase = forms[content.DefaultLang]
+	}
+	if phrase == "" {
+		return key
+	}
+	if len(args) == 0 {
+		return phrase
+	}
+	filled := make([]any, len(args))
+	for i, a := range args {
+		filled[i] = a
+	}
+	return fmt.Sprintf(phrase, filled...)
+}
+
+// numerals is an integer in the script of a language: 12 in English, ১২ in Bengali.
+func numerals(lang string, n int) string {
+	return localiseDigits(lang, strconv.Itoa(n))
+}
+
+// day is a date as a reader of that language would read it — "8 March 2026", "৮ মার্চ ২০২৬".
+//
+// One path for every language rather than [time.Time.Format] for the default locale and a hand-built
+// string for the rest, so English cannot drift from the others. The machine-readable form belongs in a
+// datetime attribute and stays ASCII (ADR-0034).
+func day(lang string, t time.Time) string {
+	if t.IsZero() {
+		return ""
+	}
+	names, ok := months[lang]
+	if !ok {
+		names = months[content.DefaultLang]
+	}
+	return strings.Join([]string{
+		numerals(lang, t.Day()),
+		names[t.Month()-1],
+		numerals(lang, t.Year()),
+	}, " ")
+}
+
+// localiseDigits swaps ASCII digits for a script's own, leaving everything else alone.
+func localiseDigits(lang, s string) string {
+	set, ok := digits[lang]
+	if !ok {
+		return s
+	}
+	out := []rune(s)
+	for i, r := range out {
+		if r >= '0' && r <= '9' {
+			out[i] = set[r-'0']
+		}
+	}
+	return string(out)
+}