docs(spec): resolve v0.7 review contradictions and fill gaps

Hard contradictions:
- Integrate `sum` into the type model: §6.1 (fourth kind), new §6.4A, §6.8
  non-Copy list, §14.5 Managed/Struct bound lattice, §9.1 variant constructors,
  §11.2 fresh sources. (sum is HirTypeKind::Sum in the compiler.)
- Receiver-call §14.6.1: the receiver binds the FIRST parameter by position
  (matches the checker), not a literal `self`; fix the self:/builder: examples.
- §14.2: define the `fresh Result<...>` / `fresh Option<...>` wrapper return
  mode used by the §18.2 composition APIs.
- §10.5: state the Copy-parameter effect rule (omit, except a receiver-call
  receiver, which declares read) — explains Int.to_string vs String.from_int.

Staleness: add String.from_int to §18.2; fix §20.1 D dangling §17.2 ref
(-> §17.1.1); version-neutral runtime pin (was rssc 0.5.x); IOError -> FileError;
List.join<T> -> List.join(List<String>) (matches stdlib); literal fresh source.

Gaps filled: integer overflow semantics (§6.8), `==`/Eq + no identity equality
(§14.6), let-mut rebinding/shadowing (§7.1), multi-feature `features:` syntax
(§15.1), await/? precedence (§14.4), program entry-point signatures (§14.9), and
a lexical-structure + grammar appendix (Appendix A).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: olwang