persistence: multi-version reads across every store

ProcessJobStore and SubstrateStore already implemented the
loader-accepts-any-version-in-[1, kVersion] pattern.  The other five
stores (ControlJobStore, CarrierStore, LoadPortStore, ExceptionStore,
SpoolStore) used strict `header[1] != kVersion` rejection, meaning
a future kVersion bump there would silently nuke every persisted
record on first replay.  That's a footgun the test_persistence_upgrade
test already flagged as a tripwire.

This commit flips the strict checks to `< 1 || > kVersion`, mirroring
PJ + Substrate.  No format change (kVersion stays at 1 across the
five stores), but:

- Future v2 of any store now Just Works: add fields at the end of
  write_record_, bump kVersion to 2, gate the new reads behind
  `if (version >= 2)`.  Old v1 records on disk continue to replay
  with the new fields defaulted.
- Future versions beyond kVersion still get rejected (downgrade
  protection — older code can't try to decode trailers it doesn't
  understand).

Comment blocks on each kVersion declaration now describe the upgrade
discipline so the next contributor doesn't reinvent it.

Test additions:
- Positive test that v1 ControlJob records load on current code
  (will continue to pass when kVersion bumps to 2, proving v1 is
  still readable)
- ExceptionStore rejects a v9 (future) record, matching CJ + Carrier
- The existing tripwire tests get retitled from "rejects unknown
  version" to "rejects a future version" to reflect the new contract

README §6 gets honest: every store is now multi-version-aware, not
just PJ + Substrate.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

include/secsgem/gem/store/spool.hpp

@@ -189,14 +189,17 @@ class SpoolStore {
     std::filesystem::path path;  // empty when persistence is disabled
   };
 
-  // Journal record layout (little-endian-free; all-bytes are explicit):
+  // Journal record (v1) layout (little-endian-free; all bytes explicit):
   //   [u8 magic = 0xE5]
-  //   [u8 version = 1]
+  //   [u8 version]
   //   [u8 stream]
   //   [u8 function]
   //   [u8 reply_expected]
   //   [u32 body_length, big-endian]
   //   [body_length bytes]
+  //
+  // Upgrade discipline: loader accepts any version in [1, kVersion];
+  // future fields append behind an `if (version >= N)` gate.
   static constexpr uint8_t kMagic   = 0xE5;
   static constexpr uint8_t kVersion = 0x01;
 
@@ -241,7 +244,8 @@ class SpoolStore {
     if (!in) return std::nullopt;
     uint8_t hdr[5];
     in.read(reinterpret_cast<char*>(hdr), sizeof(hdr));
-    if (!in || hdr[0] != kMagic || hdr[1] != kVersion) return std::nullopt;
+    if (!in || hdr[0] != kMagic ||
+        hdr[1] < 1 || hdr[1] > kVersion) return std::nullopt;
     uint8_t blen_be[4];
     in.read(reinterpret_cast<char*>(blen_be), sizeof(blen_be));
     if (!in) return std::nullopt;