feat: add GraphCache — write-through MemoryStore with SQLite persistence

[jamestexas]

Summary

• Adds graph.GraphCache — a thread-safe MemoryStore wrapper with automatic SQLite write-through persistence via ExportSQLite/ImportSQLite
• Provides tree manipulation helpers: PutDir, PutFile, AppendChild, RemoveChild, ClearChildren, plus read methods and a Store() escape hatch for batch operations
• Extracted from x-ray's SchemaCache pattern (808 LOC domain-specific cache) — this is the generic ~240-line core that any mache consumer can reuse

Motivation

x-ray's internal/api/schemacache.go mixes generic caching logic (mutex + MemoryStore + persist-on-write) with domain-specific code (URL keys, fingerprints, NavSections). Extracting the generic part into mache means:

• x-ray can compose GraphCache with its domain logic instead of reimplementing the plumbing
• Other consumers (hotsheaf, cairn, any tool caching structured data) get the same pattern for free

API

c := graph.NewGraphCache("/path/to/cache.db") // or "" for in-memory
c.PutDir("root", true)
c.PutFile("root/key", []byte("value"))
c.AppendChild("root", "root/key")

data, ok := c.GetData("root/key")       // read
children, ok := c.ListChildren("root")  // list

store := c.Store()  // escape hatch for batch ops
// ... mutate store directly ...
c.Persist()         // flush once

Test plan

• 14 tests in graph/cache_test.go covering CRUD, SQLite persistence reload, dedup, concurrency, escape hatch
• All existing graph/ tests still pass
• Race detector clean (go test -race)
• All pre-commit hooks pass (gofumpt, go vet, golangci-lint)

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