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>

examples/library/unicode.ludic

@@ -0,0 +1,49 @@
+# unicode.ludic — Unicode.* correctness over UTF-8. Byte counting is wrong for
+# non-ASCII, so we assert the code-point / grapheme invariants that must hold:
+# len counts code points not bytes, validation accepts good UTF-8, char access
+# never splits a character, case mapping round-trips, truncate keeps whole
+# characters, and grapheme_len collapses combining marks, ZWJ emoji, and flags.
+# Running it prints: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
+program Unicode {
+  entry {
+    # code points vs bytes
+    if Unicode.len("héllo") == 5 { print(1) }
+    if Unicode.byte_len("héllo") == 6 { print(2) }
+    if Unicode.len("abc") == 3 { print(3) }
+
+    # validation of well-formed UTF-8
+    if Unicode.is_valid_utf8("héllo") { print(4) }
+
+    # code-point access by index (é is U+00E9 = 233), out of range -> -1
+    if Unicode.char_at("héllo", 0) == 104 { print(5) }
+    if Unicode.char_at("héllo", 1) == 233 { print(6) }
+    if Unicode.char_at("héllo", 5) < 0 { print(7) }
+
+    # chars() yields every code point in order
+    let cs = Unicode.chars("héllo")
+    if len(cs) == 5 { print(8) }
+    if cs[1] == 233 { print(9) }
+
+    # case mapping (ASCII + Latin-1) round-trips
+    if Unicode.upper("héllo") == "HÉLLO" { print(10) }
+    if Unicode.lower("HÉLLO") == "héllo" { print(11) }
+
+    # truncate keeps whole characters: first 3 code points of "héllo" is "hél"
+    if Unicode.truncate("héllo", 3) == "hél" { print(12) }
+
+    # grapheme clustering: ASCII is one grapheme per code point
+    if Unicode.grapheme_len("abc") == 3 { print(13) }
+
+    # a combining accent joins its base: "café" decomposed is 5 code points but
+    # 4 user-perceived characters
+    if Unicode.grapheme_len("café") == 4 { print(14) }
+    if Unicode.len("café") == 5 { print(15) }
+
+    # a ZWJ emoji sequence (family) is 5 code points but one grapheme
+    if Unicode.grapheme_len("👨‍👩‍👧") == 1 { print(16) }
+    if Unicode.len("👨‍👩‍👧") == 5 { print(17) }
+
+    # a flag is two regional indicators but one grapheme
+    if Unicode.grapheme_len("🇺🇸") == 1 { print(18) }
+  }
+}