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>

test.sh

@@ -68,8 +68,8 @@ if ./selfhost/game-build.sh build/ludicc examples/toggle.ludic "/tmp/ludic_tog"
 else bad "toggle: $(tail -1 /tmp/tog.out)"; fi
 # String operators: ==/!= compare by content, + concatenates (str_* prelude).
 if ./selfhost/game-build.sh build/ludicc examples/strings.ludic "/tmp/ludic_str" >/tmp/str.out 2>&1 \
-   && [ "$(/tmp/ludic_str </dev/null | tr '\n' ' ')" = "1 2 3 4 5 " ]; then
-  ok "strings.ludic (str ==/!=/+ operators)"
+   && [ "$(/tmp/ludic_str </dev/null | tr '\n' ' ')" = "1 2 3 4 5 6 7 " ]; then
+  ok "strings.ludic (str ==/!=/+ and `interpolation`)"
 else bad "strings: $(tail -1 /tmp/str.out)"; fi
 # --- Toolchain-agent CLI smoke tests append below this line ---
 echo "== self-hosted front-end binaries (ludicc / ludic) =="