test: add partial_settle dual-caller and snapshot-capture coverage

[tebney]

Summary

Closes Liquifact#408.

Adds the missing partial_settle test scenarios required by the issue:

Test	Scenario
test_partial_settle_snapshot_not_overwritten	Snapshot is stable after close; second call panics on the status guard
test_partial_settle_investor_payout_via_snapshot	compute_investor_payout produces correct pro-rata payout using the early snapshot after partial_settle + settle
test_partial_settle_auth_required	require_auth() is enforced — verified via mock_auths(&[])

The following tests were already present and provide the remaining coverage:

• SME caller succeeds (test_partial_settle_sme_happy_path)
• Admin caller succeeds (test_partial_settle_admin_happy_path)
• Third-party caller panics (test_partial_settle_unauthorized_caller_panics)
• Legal hold blocks (test_partial_settle_blocked_by_legal_hold)
• Non-open status blocks (test_partial_settle_rejected_if_not_open)
• Snapshot values correct (test_partial_settle_writes_correct_snapshot)
• Funding blocked after close (test_funding_blocked_after_partial_settle)

Docs

docs/escrow-snapshot.md — added partial_settle and Early Snapshot Capture section cross-linking all 9 test scenarios and documenting the write-once guarantee.

Security notes

• Only SME or Admin can call partial_settle (require_auth() + address check).
• Snapshot is write-once: the if !has(FundingCloseSnapshot) guard plus the status == 0 pre-condition together make overwrite impossible.
• No production code changes were required.