Fix masternode collateral vin validation

[NateIsern]

Motivation

• Prevent a collateral validation bug that accepted unowned outputs by scanning any output in the funding transaction and by using a 4999.99 FAIR dummy threshold instead of ensuring the referenced outpoint is the exact 5000 FAIR collateral output.

Description

• Require the referenced vin.prevout.n output itself to match MASTER_NODE_AMOUNT * COIN and pay to the collateral pubkey by changing IsVinAssociatedWithPubkey() to inspect txVin.vout[vin.prevout.n] instead of scanning all outputs. (src/obfuscation.cpp)
• Restore the dummy collateral checks to use the exact MASTER_NODE_AMOUNT * COIN amount instead of 4999.99 * COIN in masternode liveness, broadcast admission, and legacy dsee paths. (src/masternode.cpp, src/masternodeman.cpp)
• Commit message: Fix masternode collateral vin validation.

Testing

• Ran git diff --check with no issues reported. (succeeded)
• Attempted ./autogen.sh in this environment and it failed due to missing aclocal (environment dependency), so a full build/test could not be executed here. (failed due to environment)
• Verified the modified lines via repository search (rg) and inspected the resulting diffs to confirm the intended changes were applied. (inspection only)

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Codex Task