Make versioning backend-agnostic

[xxfast]

VersionedCodec lived in kstore-file and did its own file IO, so kstore-storage couldn't have versioned stores.

Moved it into kstore as a decorator over three codecs:

• data - the current shape of T
• versionCodec - persists the version alongside the data
• staleCodec - re-reads the old payload in a raw shape that survives schema changes

Versioning is itself just persistence, so it's delegated to codecs. Nothing in VersionedCodec knows about files or keys.

What this gets us

kstore-storage gets storeOf(key, version, ...) for free. 48 lines pointing StorageCodec at a "$key@version" sibling record.

kstore-file keeps storeOf(file, version, ...) unchanged. It's now a thin factory over FileCodec, net -44 lines.

Behaviour

• Data is written first, version second. A crash in between leaves the new data with a stale version, which still decodes directly. The other order would claim a version the data was never written to.
• A missing version record reads as version 0, ie the store predates versioning.
• A version record that exists but can't be read leaves version as null, the same way an unreadable payload leaves previous as null.

API

Migration<T, R> is now generic over the raw shape. kstore-file keeps its Migration<T> typealias and DefaultMigration so nothing downstream breaks. apiDump is committed.

Tests

• VersionedCodecTests (10) runs the decorator against an in-memory MapCodec, proving it's backend-agnostic
• kstore-file KVersionedStoreTests (14), behaviour unchanged
• kstore-storage KVersionedStoreTests (6), new

Docs updated with the storage factory and the custom codec path.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 95.12195% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.52%. Comparing base (f754f1b) to head (dd73ab3).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...b/xxfast/kstore/file/extensions/KVersionedStore.kt	90.90%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #179      +/-   ##
==========================================
- Coverage   94.35%   93.52%   -0.83%     
==========================================
  Files           8        9       +1     
  Lines         124      139      +15     
  Branches       11       12       +1     
==========================================
+ Hits          117      130      +13     
- Misses          0        2       +2     
  Partials        7        7              

Files with missing lines	Coverage Δ
.../github/xxfast/kstore/extensions/VersionedCodec.kt	100.00% <100.00%> (ø)
...b/xxfast/kstore/file/extensions/KVersionedStore.kt	93.93% <90.90%> (-6.07%)	⬇️