[upstream-sync] Support attachment providers for target states (upstream PR #1745)

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Upstream Change Summary

Type: architectural-change
Difficulty: Medium
Recommendation: Adopt

Upstream introduced "attachment providers" for target states in rust/core/src/engine/target_state.rs. This is an architectural extension to the target state subsystem (which already tracks destructive/lossy changes per upstream PR #1736, tracked in recoco as #36). Attachment providers allow targets to supply additional state metadata during execution.

Upstream References

• PR: feat: support attachment providers for target states cocoindex-io/cocoindex#1745
• Release: v1.0.0-alpha27
• Upstream repo: https://github.com/cocoindex-io/cocoindex

Relevant Upstream Files / Areas

• rust/core/src/engine/target_state.rs — attachment provider trait and integration
• rust/py/src/target_state.rs — Python binding layer (exclude from recoco)

Recoco Considerations

• Maps to crates/recoco-core engine/target_state module
• Exclude the Python binding (rust/py/src/target_state.rs) entirely
• This builds on the target states work tracked in [upstream-sync] Track destructive and lossy changes for target states (alpha25 architectural change) #36 — should be integrated after or alongside that work
• Review whether this affects the public ops::interface trait definitions for targets
• Consider if this should be exposed in recoco's public target connector API

Integration Notes

This is part of a larger set of target state architectural changes in the v1 alpha series. Related upstream changes include #1739 (reprocessing on target_id/schema_version) and #1740 (child_invalidation on all targets) — both of which are Python-connector-only but conceptually related. This specific change has Rust core impact and should be adopted.