feat: remove stablecoin exchange executeBlock system tx post AllegroModerato

[fgimenez]

Closes #1435

Simplifies the stablecoin DEX by making orders immediately active when place() or place_flip() is called, eliminating the need for the executeBlock system transaction to finalize pending orders.

• Orders are now linked directly into the orderbook when placed (post AllegroModerato)
• executeBlock() becomes a no-op post AllegroModerato
• Payload builder skips the stablecoin exchange system tx post AllegroModerato
• Pre-hardfork behavior is preserved for backward compatibility

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Project	Deployment	Review	Updated (UTC)
tempo-docs	Ready	Preview, Comment	Dec 15, 2025 3:21am

[0xKitsune]

Closing with preference for #1509