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/lang/strings.ludic

@@ -1,5 +1,5 @@
 # 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
+# interpolate with `backticks {expr}`. Running it prints: 1 2 3 4 5 6 7 8 9
 program Strings {
   entry {
     let greeting = "hello"
@@ -13,7 +13,7 @@ program Strings {
     # build up a string in a loop
     var line = ""
     var i = 0
-    while i < 3 { line = line + "ab"; i = i + 1 }
+    while i < 3 { line += "ab"; i += 1 }
     if line == "ababab" { print(4) }
 
     if (greeting + who) != greeting { print(5) }