feat: support auto-generated _k keys in range item matching

[adnaan]

Summary

• Check for auto-generated _k field first in getItemKey() to support range items without explicit key attributes

Context

This is the client-side counterpart to livetemplate/livetemplate#108.

When templates don't define explicit key attributes (like data-key="{{.ID}}"), the server now injects a content-based hash into each item's _k field. This change ensures the client checks _k first, allowing proper key matching.

Test plan

• All 193 existing tests pass
• Works with server-side auto-key injection

🤖 Generated with Claude Code

[claude]

LGTM! Clean implementation of the auto-generated key support.

The priority order makes sense: _k field (server-injected hash) takes precedence over explicit key attributes. This ensures backward compatibility while supporting the new server-side auto-key injection.

No issues found:

• Proper type check for _k as string
• Early return pattern is efficient
• Maintains existing fallback logic for explicit keys

Approved ✓

[Copilot]

Pull request overview

This PR adds client-side support for auto-generated _k keys in range item matching, complementing the server-side auto-key injection feature from livetemplate/livetemplate#108. When templates don't define explicit key attributes, the server now injects content-based hashes into each item's _k field, and this change ensures the client checks that field first for proper item identification in range operations.

Changes:

• Modified getItemKey() to prioritize the auto-generated _k field over explicit key attributes from rangeIdKeys

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