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>

docs/language/unicode/unicode-is_valid_utf8.md

@@ -0,0 +1,25 @@
+---
+id: unicode-is_valid_utf8
+name: Unicode.is_valid_utf8
+category: unicode
+kind: namespace-method
+tokens: Unicode.is_valid_utf8
+sig: Unicode.is_valid_utf8(s) -> bool
+tip: Is a string well-formed UTF-8?
+order: 3
+ns: Unicode
+member: is_valid_utf8
+---
+
+Strictly validates that the bytes are well-formed UTF-8: correct continuation bytes, no overlong encodings, no surrogate code points, and nothing above U+10FFFF. Use it as a gate on untrusted input — a chat line, a downloaded file, a mod's data — before treating it as text.
+
+Parameters:
+- `s` — the bytes to validate
+
+```ludic
+program Valid {
+  entry {
+    if Unicode.is_valid_utf8("safe text") { print(1) } else { print(0) }
+  }
+}
+```