feat(stdlib): per-decl nominal identity + same-name shadows — close 0105 [stdlib E2]

Make same-name top-level types in different sources DISTINCT nominal types
instead of collapsing last-wins in the type table (issue 0105).

Registration:
- internNamedTypeDecl assigns a per-decl nominal_id and populates
  type_decl_tids. The first author of a name keeps nominal_id 0 (byte-identical
  to pre-E2); a genuine cross-module shadow (>=2 distinct normalized-path
  authors per the import facts) gets a fresh id -> a distinct TypeId.
- mergeFlat/addOwnDecl stop first-wins-dropping per-source decls (named types +
  non-fn const_decls) so every same-name author reaches registration; functions
  and var_decls (incl. #foreign extern globals) keep first-wins.

Resolution (selectNominalLeaf):
- own-author wins; else flatTypeAuthorCount over the transitive flat closure:
  >=2 distinct -> .ambiguous (loud diagnostic + poison); exactly one -> resolved;
  a flat author not yet findByName-registered -> .undeclared stub (not a leak).
- struct-literal type names route through the same source-aware leaf.
- lazyLowerFunction pins the function's own source before resolving its return
  type, so a shadowed signature type resolves in its module, not the caller's.

Codegen:
- mangleTypeName appends __n<id> for nonzero nominal_id so same-name shadows get
  distinct monomorph symbols (struct_to_string__Box vs __Box__n1).

Library hygiene:
- rename trace.sx's compiler-contracted Frame -> TraceFrame (+ the two compiler
  findByName sites) so it never collides with a UI/geometry Frame; the layout is
  structural (getFrameStructType / SxFrame), name-independent.

Examples: 0752-0756 pin the five 0105 cases (distinct fields / same fields /
own-wins / ambiguous / alias per-source); 0170 pins the folded anon-struct-field
regression.

examples/0752-modules-same-name-struct-distinct-fields/a.sx

@@ -0,0 +1,3 @@
+// Module A authors its OWN `Box` (one `s64` field `x`).
+Box :: struct { x: s64; }
+a_box :: () -> Box { return Box.{ x = 7 }; }

examples/0752-modules-same-name-struct-distinct-fields/b.sx

@@ -0,0 +1,5 @@
+// Module B authors a DIFFERENT `Box` (two fields `p`, `q`) — a same-name shadow
+// of A's `Box`. Pre-0105 the two collapsed last-wins in the type table, so one
+// module's field set vanished; now each `Box` is a distinct nominal type.
+Box :: struct { p: s64; q: s64; }
+b_box :: () -> Box { return Box.{ p = 3, q = 4 }; }