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>

examples/strings.ludic

@@ -1,5 +1,5 @@
-# strings.ludic — strings are values: compare with ==/!=, join with +.
-# Running it prints: 1 2 3 4 5
+# strings.ludic — strings are values: compare with ==/!=, join with +, or (best)
+# interpolate with `backticks {expr}`. Running it prints: 1 2 3 4 5 6 7
 program Strings {
   entry {
     let greeting = "hello"
@@ -17,5 +17,10 @@ program Strings {
     if line == "ababab" { print_int(4) }
 
     if (greeting + who) != greeting { print_int(5) }
+
+    # interpolation: `{expr}` embeds any expression (numbers become text)
+    let n = 42
+    if `{greeting}, {who}!` == "hello, Ludic!" { print_int(6) }
+    if `n={n}, next={n + 1}` == "n=42, next=43" { print_int(7) }
   }
 }