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_decl.ludic

@@ -109,6 +109,7 @@ function emit_program() -> void {
   if g_uses_noisert { emit_noise_prelude() }     # @fn_noise_value2/perlin2/simplex2/fbm2/cellular2 (Q16.16)
   if g_uses_logrt { emit_log_prelude() }         # @L_log_level + @fn_log_emit (levelled stderr sink)
   if g_uses_osrt { emit_os_prelude() }           # @fn_os_args/platform/arch/save_dir/... (libc env + uname)
+  if g_uses_unicodert { emit_unicode_prelude() } # @fn_uni_len/valid/decode/case/truncate/grapheme (UTF-8)
   if g_uses_datert { emit_datetime_prelude() }   # @fn_days_from_civil / @fn_civil_from_days conversions
 }