feat(compiler): list literals, typed compound assignment, file:line diagnostics

- `[a, b, c]` list literals (E_LIST → emit_list); static_type learns
  slice-element, `new T`, list, string and literal kinds
- `x op= y` lowers through the same path as `x = x op y` (emit_bin_vals):
  fixed `*=`/`/=` use the Q16.16 64-bit paths, string `+=` concatenates,
  int→long widens; unary `-` keeps a fixed operand's type (arith_ty)
- one `unescape()` table for "strings", 'chars' and `interpolation`;
  `'\''`, `'\\'`, `'\"'` no longer read as 0; unterminated char literals
  and unexpected characters are errors instead of silently skipped
- every diagnostic is `file:line: error: msg` (g_parse_file / g_err_file,
  Node.file + Node.line set by node()); tok_desc() in expectation errors;
  duplicate `function` names and unknown `phase` names are reported in
  source terms (phase_id used to default unknown phases to Overlay)
- interpolation holes skip braces inside string literals
- hand-IR preludes move from the user `@fn_` prefix to `@lp_` so a user
  `is_ws` / `str_eq` / `path_join` no longer collides at link time
- `@ClearColor(expr)` accepts any constant expression; `Os.pid()` added
  (docs page + inventory); `str_starts()` in support/str
- main.ludic: `else if` flag ladder, char literals, stale script comments
- examples/lang/operators.ludic covers all of the above; os.ludic covers
  Os.pid; docs pages for Os.pid and the Overlay phase; ten changesets
- reseeded: selfhost/ludicc.seed.ll is the new compiler's own fixpoint

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

examples/library/os.ludic

@@ -2,7 +2,7 @@
 # host input, so instead of fixed answers we assert the invariants that must hold
 # regardless of where it runs: an env var round-trips, env_or falls back when a
 # var is unset, unset removes it, args() agrees with arg_count(), and the host
-# facts and known folders are all non-empty. Running it prints: 1 2 3 4 5 6 7 8 9 10 11 12 13
+# facts and known folders are all non-empty. Running it prints: 1 2 3 4 5 6 7 8 9 10 11 12 13 14
 program Os {
   entry {
     # environment variables round-trip through set/get
@@ -30,8 +30,11 @@ program Os {
     if len(Os.save_dir("MyGame")) > 0 { print(11) }
     if len(Os.temp_dir()) > 0 { print(12) }
 
+    # the process id is a live, positive OS handle
+    if Os.pid() > 0 { print(13) }
+
     # diagnostics go to stderr, so they never disturb this program's stdout
     Os.stderr_write("os.ludic: hello from stderr\n")
-    print(13)
+    print(14)
   }
 }