feat(stdlib): add Unicode.* — UTF-8 code points, graphemes, case mapping (#13)

Make Ludic text correct-by-default over UTF-8, so player names, translated UI,
and chat behave for every language instead of counting bytes and splitting
characters in half. The byte-oriented Text.* stays for speed; Unicode.* is the
layer that understands code points and (approximately) grapheme clusters.

  - len / byte_len          code points vs bytes — the two lengths, kept distinct
  - is_valid_utf8           strict validation of untrusted input
  - char_at / chars         code-point access by index; chars() -> []int
  - upper / lower           case mapping (ASCII + Latin-1)
  - truncate                first n code points, never a half-character
  - grapheme_len            user-perceived characters (approx UAX#29)

Pure integer/byte IR over NUL-terminated buffers; C-free, no data-table blob.
Decoding and validation cover the full UTF-8 range (overlong/surrogate/>10FFFF
rejected). grapheme_len collapses combining marks, variation selectors, ZWJ
sequences (family emoji), and regional-indicator flag pairs. Documented v1
scope: wider-script/locale case rules (Latin-Extended, Greek, Cyrillic, Turkish
i, German ß) and NFC normalization are follow-ups.

- examples/library/unicode.ludic: asserts the invariants across ASCII, Latin-1
  (é round-trips through upper/lower), a decomposed "café" (5 code points, 4
  graphemes), a ZWJ family emoji (5 code points, 1 grapheme), and a flag (2
  regional indicators, 1 grapheme). Wired into `x test` (now 55 passed).
- docs: a new Unicode section + 9 per-symbol pages clarifying byte vs code point
  vs grapheme; inventory updated; every fence passes check-docs; site builds.
- seed regenerated; `x bootstrap-cfree` fixpoint holds.

Closes #13

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

selfhost/emit_expr.ludic

@@ -251,6 +251,10 @@ function emit_ns_call(ns: pointer, meth: pointer, e: Node) -> Val {
     if is_os_ns(meth) { return emit_os_ns(meth, e) }
     perr(`unknown builtin Os.{meth}`)
   }
+  if (ns == "Unicode") {
+    if is_unicode_ns(meth) { return emit_unicode_ns(meth, e) }
+    perr(`unknown builtin Unicode.{meth}`)
+  }
   if (ns == "Vector") {
     if is_vector_ns(meth) { return emit_vector_ns(meth, e) }
     perr(`unknown builtin Vector.{meth}`)