Phase 7c: string operators + and ==/!= (retire streq/sconcat)

Strings are values now: `a + b` concatenates and `a == b` / `a != b` compare by
content, replacing the 605 `sconcat(...)` / `streq(...)` calls that made the
compiler read like C.

  streq(name, "let")            -> name == "let"
  sconcat("load ", reg)         -> "load " + reg
  sconcat(a, sconcat(b, c))     -> a + b + c

Implementation: emit_bin gains a string path. Strings are pointer-typed, so any
`+` with a pointer operand concatenates and `==`/`!=` between pointers compares
content — except when one side is the `null` literal, which stays a pointer
identity test (the only two kinds of pointer `==` in the codebase). Both call a
small hand-written IR prelude, @fn_str_eq / @fn_str_concat, emitted once into any
program that uses string ops (so it works for tools, games and the compiler with
no runtime-splice dependency and no duplicate symbols).

Delivered as two reseeds: (A) add the operators + prelude with the full
pointer-aware dispatch, keeping streq/sconcat; (B) migrate every call site
(354 lines, via a string-literal-safe balanced-paren script that leaves the
function definitions alone) and delete streq/sconcat. examples/strings.ludic +
a test.sh smoke (prints 1 2 3 4 5) guard it.

Reseeded (21890 lines); C-free fixpoint holds; goldens byte-identical; 18/18;
vocab + doc-fences clean.

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

examples/strings.ludic

@@ -0,0 +1,21 @@
+# strings.ludic — strings are values: compare with ==/!=, join with +.
+# Running it prints: 1 2 3 4 5
+program Strings {
+  entry {
+    let greeting = "hello"
+    if greeting == "hello" { print_int(1) }         # content comparison
+    if greeting != "world" { print_int(2) }
+
+    let who = "Ludic"
+    let msg = greeting + ", " + who + "!"            # concatenation chains
+    if msg == "hello, Ludic!" { print_int(3) }
+
+    # build up a string in a loop
+    var line = ""
+    var i = 0
+    while i < 3 { line = line + "ab"; i = i + 1 }
+    if line == "ababab" { print_int(4) }
+
+    if (greeting + who) != greeting { print_int(5) }
+  }
+}