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/frontend/ast.ludic

@@ -1,5 +1,5 @@
 # ast.ludic — the node kinds and the single Node shape they share, plus the
-# declaration tables the emitter reads. Mirrors compiler/front/ast.h + sem/.
+# declaration tables the emitter reads.
 
 # declarations
 const N_STRUCT: int = 0
@@ -36,6 +36,7 @@ const N_PREFAB: int = 53         # prefab Name: Model { Comp { field: v }, … }
 const E_TRY:    int = 52         # try EXPR else { ... } — recover a fallible result as a value (issue #46)
                                  #   a=the fallible (result-typed) expression  b=else block (its
                                  #   trailing expression is the fallback)  line=source line
+const E_LIST:   int = 54         # [a, b, c] — a slice literal; kids=the elements, all of one type
 # statements
 const S_LET:      int = 10
 const S_ASSIGN:   int = 11
@@ -81,11 +82,16 @@ property Node {
   c:    Node
   kids: []Node                # variadic children
   line: int = 0
+  file: pointer = null        # the source file the node was parsed from (for diagnostics)
 }
 
+# every node remembers where it was parsed (file + the line of the token the
+# parser is looking at), so any later diagnostic can point at real source
 function node(kind: int) -> Node {
   let n = new Node
   n.kind = kind
   n.kids = new []Node
+  n.file = g_parse_file
+  if g_parsing and pi < len(toks) { n.line = toks[pi].line }
   return n
 }