chore: remove KEY_DATA migration code after next release

[sanity]

Context

PR #2814 migrated the store indexes (contracts, delegates, secrets) from the custom KEY_DATA append-only file format to ReDb. This fixes a stale file handle bug where data was lost after background compaction.

The PR includes migration code that:

1. Reads existing KEY_DATA files on startup
2. Writes all entries to the new ReDb tables
3. Renames KEY_DATA to KEY_DATA.migrated

Task

After the next release (giving users time to upgrade and migrate their data), remove the migration code:

Files to clean up

1. crates/core/src/wasm_runtime/contract_store.rs

   • Remove migrate_from_legacy() function
   • Remove call to migration in new()
   • Remove legacy_key_data_path from struct if unused

2. crates/core/src/wasm_runtime/delegate_store.rs

   • Remove migrate_from_legacy() function
   • Remove call to migration in new()

3. crates/core/src/wasm_runtime/secrets_store.rs

   • Remove migrate_from_legacy() function
   • Remove call to migration in new()

4. crates/core/src/wasm_runtime/store.rs

   • Remove or mark as dead code the legacy SafeWriter, KeyDataReader, and related structs/functions that were only kept for migration
   • Remove the KEY_DATA constant if unused elsewhere

Verification

Before removing migration code, verify that:

• At least one release has shipped with the migration code
• Users have had reasonable time to upgrade (check release date)
• No reports of migration failures

[AI-assisted - Claude]

[github-actions]

🤖 Auto-labeled

Applied labels:

• T-enhancement (95% confidence)
• E-medium (80% confidence)
• A-contracts (90% confidence)

Reasoning: This is a post-release code cleanup/removal of legacy migration logic (not a bug report or new feature), so T-enhancement is appropriate. The change touches multiple wasm_runtime store files and requires verification that a release with the migration shipped, so I estimate medium effort (several small code edits plus release verification). The affected code is in contract/delegate/secrets stores (wasm runtime), so A-contracts is the most relevant area.

Previous labels: none

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