Problem: During IBD/reindex, ConnectBlock() skips script verification for blocks in the assumevalid period.
However, it still performs one heap allocation for a fresh txsdata vector and default-constructs one PrecomputedTransactionData entry for every transaction in each connected block.
Because CheckInputScripts() is not called in that path, that work is unused and only adds allocator churn and avoidable heap fragmentation on large blocks.
Fix: Leave txsdata empty unless fScriptChecks is true, then resize it before queued checks can take pointers into the vector.
Update the lifetime comment and add an indexed-use assert that matches the real precondition.
Reproducer: On a synthetic ~7000-transaction block, the skipped-check path avoids the unused heap allocation and saves ~2MB memory per block.
<details><summary>ConnectBlockAllSchnorr mem usage: 7000 txs, num_schnorr=1, num_ecdsa=0, fScriptChecks=false</summary>
for C in 1835f2fcbf482035e1c11c72ae39cee4e24e4c00 832dc594f72bb6e8415674dce434684202073b12; do
git reset --hard -q && git checkout -q "$C"
perl -0pi -e 's/(num_txs = )1000/${1}7000/; s@(/\*num_schnorr=\*/)\d+, (/\*num_ecdsa=\*/)\d+@${1}1, ${2}0@; s/(fScriptChecks)\{[^}]+\};/${1}{false};/;' src/bench/connectblock.cpp src/validation.cpp
cmake -B build -DBUILD_BENCH=ON >/dev/null 2>&1 && cmake --build build --target bench_bitcoin >/dev/null 2>&1
/usr/bin/time -v build/bin/bench_bitcoin -filter=ConnectBlockAllSchnorr -sanity-check 2>&1 >/dev/null | grep 'Maximum resident'
done
1835f2fcbf ... Maximum resident set size: 58704 kB 832dc594f7 ... Maximum resident set size: 56664 kB
</details>
