fix: enable pool on TTD during initial sync

[isuniverseok-ua]

PR description

The issue was because we only enabled the txn pool after initial sync completed. In a Holesky shadow fork, initial sync can remain “not done” even after Merge, so the pool stayed disabled while we still imported/produced blocks. Result: empty blocks despite network activity. To fix this I enabled the txn pool as soon as TTD is reached (post-merge), even if initial sync isn’t complete. Now also I subscribe to the 'TTD-reached' event so the pool flips on immediately when that happens and keeping P2P txn gossip handlers gated on “in-sync” exactly as before.

Why this fixes the issue

After TTD, block building relies on the tx pool. With the pool enabled, locally submitted transactions can be accepted and included, so we stop producing empty blocks during the shadow fork window. Gossip stays off until we’re in-sync, so we don’t risk propagating/accepting out-of-sync traffic. Safety is unchanged; we only allow local acceptance and block inclusion.

Fixed Issue(s)

fixes #9240

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