test(escrow): snapshot immutability and over-funding capture

[Edoscoba]

Summary

Adds tests proving FundingCloseSnapshot is created only when funding first crosses the target, captures the full over-funded funded_amount, records the close ledger timestamp/sequence, and remains immutable after later lifecycle actions or rejected same-ledger funding attempts.

Also updates snapshot documentation and rustdoc to clarify the write-once pro-rata denominator invariant.

Validation:

• cargo test
• cargo clippy -p liquifact_escrow --all-targets -- -D warnings
• cargo llvm-cov --features testutils --fail-under-lines 95 --summary-only -p liquifact_escrow

close #258

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