feat(sdkx): checkpoint backends (sqlite/postgres) + image-LLM OTel parity

[lIang70]

Summary

Phase 1 of the sdkx track that closes the v0.2 roadmap. Two themes,
one PR (体量小、共享 `sdkx/v0.2.7` tag):

1. Concrete `engine.CheckpointStore` backends for the new
   `engine.Resumer` / `engine.LoadAndResume` APIs shipped in
   `sdk/v0.2.9`.
2. Image-LLM OTel parity — the 3 image adapters added in
   PR feat(sdkx/llm): add image-generation adapters + align chat caps #72 were missing the instrumentation every chat provider
   already ships. Closes the gap.

sdkx/engine/checkpoint/

```
├── conformance/ ← shared contract suite (Save/Load/List/Delete + concurrency)
├── sqlite/ ← modernc.org/sqlite, single-writer pool + busy_timeout
└── postgres/ ← jackc/pgx/v5 via pgxpool, JSONB + UPSERT
```

• Both backends ship `Migrate(ctx)` (auto-run by `Open`, exposed
  for `Wrap` callers) and a `WithTable` option for multi-vessel
  namespacing.
• Both implement the optional `CheckpointLister` and
  `CheckpointDeleter` interfaces.
• Postgres tests gate on `FC_PG_DSN` env var, matching the pattern
  used by `sdkx/retrieval/postgres`.

`redis` checkpoint backend was deliberately scoped out — it is a
cache tier, not a source-of-truth, and its semantics differ enough
to warrant its own minor.

Image-LLM OTel parity (`sdkx/llm/{bytedance,minimax,qwen}/image`)

Each image adapter now follows the exact pattern the chat providers
use:

```go
ctx, span := telemetry.Tracer().Start(ctx,
fmt.Sprintf("llm.qwen-image.generate.%s", l.model),
trace.WithAttributes(
attribute.String(telemetry.AttrLLMProvider, providerKey),
attribute.String(telemetry.AttrLLMModel, l.model),
))
defer span.End()
// ... call, time, RecordLLMMetrics(success/error) ...
```

Provider keys `bytedance-image` / `minimax-image` / `qwen-image`
keep image traffic distinct from chat in dashboards.
`TokenUsage.OutputTokens` carries `image_count` (the only quantity
these endpoints report).

After this PR, every `llm.LLM` in sdkx — chat or image, direct or
delegating — emits OTel spans and records metrics through the same
`sdk/telemetry` helpers.

Test plan

• `make fmt` clean
• `go vet ./...` (sdkx)
• `go test ./engine/checkpoint/...` — sqlite suite passes incl.
  16-way concurrent Save; postgres skips when `FC_PG_DSN` is unset.
• `go test ./llm/{bytedance,minimax,qwen}/image` — green.
• Postgres backend has not been exercised against a live PG
  instance in CI; suite design mirrors sdkx/retrieval/postgres which
  has the same gating.

Release notes

Cumulative bump for `sdkx/v0.2.7`. No `[skip-tag:sdkx]` marker —
this is a single-PR release.

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