v8: earlier PSE pause cutoff for testnet

[metalarm10]

Summary

Adds a per-chain PSE postpone cutoff so the PSE pause and new inflation params can be verified on testnet before mainnet. Same v8 code; only the cutoff date differs per chain.

• Mainnet: cutoff 2026-11-06 12:00 UTC (unchanged)
• Testnet: cutoff 2026-10-05 12:00 UTC (Oct 5 payout kept, Nov 5 onward moves to 2027)

Testnet behavior

• v8 upgrade after Oct 5 PSE (expected): the upgrade shifts Nov 5 onward to 2027 and sets the pause inflation params immediately.
• Upgrade before Oct 5: Oct 5 pays normally, then the EndBlocker sets the params when it completes (same path as mainnet).
• Testnet: cutoff 2026-10-05 12:00 UTC. The Oct 5 payout is the last one; there is no Nov 5, 2026 payout, and every later testnet payout moves forward one year (next: Nov 5, 2027).

Testing

Unit tests (app/upgrade/v8, simapp, real v8 upgrade handler):

• TestV8Upgrade_TestnetAfterOctober: testnet upgrade after Oct 5 shifts Nov 5 onward to 2027 and sets the pause inflation at the upgrade.
• TestV8Upgrade_TestnetBeforeOctober: testnet upgrade before Oct 5 keeps Oct 5; the EndBlocker sets the params when it completes.
• TestV8Upgrade_MainnetUnaffectedByTestnetCutoff: mainnet keeps Oct 6 and Nov 6, params not set at upgrade.
• TestPostponePSEDistributions_Testnet: testnet schedule shift with its own cutoff.
• TestPSEPostponeCutoffFor: cutoff per chain (mainnet, testnet, devnet, unknown).
• Negative controls: swapping the cutoffs between networks fails these tests.
• CI is green.

Reviewers checklist:

• Try to write more meaningful comments with clear actions to be taken.
• Nit-picking should be unblocking. Focus on core issues.

Authors checklist

• Provide a concise and meaningful description
• Review the code yourself first, before making the PR.
• Annotate your PR in places that require explanation.
• Think and try to split the PR to smaller PR if it is big.

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