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>

selfhost/tests/math2.ludic

@@ -10,11 +10,11 @@ program T {
     print(Math.floor(Math.dist2(0.0, 0.0, 3.0, 4.0)))    # 25
     print(Math.floor(Math.deg_to_rad(180.0) * 1000.0))   # 3142
     print(Math.floor(Math.rad_to_deg(3.1415926)))        # 180
-    print(Math.posmod(0 - 1, 5))                          # 4
+    print(Math.posmod(-1, 5))                          # 4
     print(Math.wrap(12, 0, 10))                            # 2
     print(Math.ping_pong(13, 10))                          # 7
     print(Math.floor(Math.snapped(2.3, 0.5) * 10.0))      # 25
     print(Math.floor(Math.move_toward(0.0, 10.0, 3.0) * 10.0))  # 30
-    print(Math.floor(Math.smoothstep(0.0, 10.0, 5.0) * 1000.0)) # 500
+    print(Math.floor(Math.smoothstep(0.0, 10.0, 5.0) * 1000.0))  # 500
   }
 }