Phase 7d: string interpolation text {expr} text

The readable way to build strings, as you noted `{a} {b}` beats `a + " " + b`.
A backtick string embeds any expression in `{…}` and desugars to the Phase-7c
`+` chain, wrapping each hole in `str(...)`:

  `hello {name}, n={count + 1}`
    ==  "hello " + name + ", n=" + str(count + 1)

- Lexer: a backtick captures its content raw as TK_INTERP.
- Parser: parse_interp splits literal runs from `{…}` holes (brace-depth aware,
  `{{`/`}}` escape to literal braces), re-lexes each hole as a full expression
  (save/restore toks/pi like an import), and folds it all into E_BIN(+) nodes —
  so no new AST or runtime beyond the existing concat.
- str(x): a string passes through; int/bool/fixed convert via a small emitted
  @fn_int_str prelude (digits from the end of a buffer, '-' for negatives),
  emitted once into any program that uses it.

examples/strings.ludic gains interpolation cases (now prints 1..7); the smoke
covers it. Grammar + ludic_syntax.h tokenize backtick strings (holes highlighted
as embedded code). LANGUAGE.md documents it as the preferred form.

Reseeded (22530 lines); C-free fixpoint holds; goldens identical; 18/18; vocab +
doc-fences clean.

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

tools/ludic-tools/ludic_syntax.h

@@ -227,6 +227,12 @@ static void lud_lex(LLex* L, const char* src){
             if (src[i] == '"') i++; else bad = 1;
             ltok_push(L, LT_STR, s, i, line, bad); continue;
         }
+        if (c == '`'){                      /* `interpolated {expr}` string */
+            int s = i; i++; int bad = 0;
+            while (src[i] && src[i] != '`'){ if (src[i] == '\\' && src[i+1]) i += 2; else i++; }
+            if (src[i] == '`') i++; else bad = 1;
+            ltok_push(L, LT_STR, s, i, line, bad); continue;
+        }
         if (c == '\''){
             int s = i; i++; int bad = 0;
             if (src[i] == '\\' && src[i+1]) i += 2; else if (src[i] && src[i] != '\n') i++;