fix: cap governance timelocks

[carrion256]

Summary

Fixes governance timelock-model findings in governance-control-plane:

• A-040 / Nexus 890b1bbf-ea61-47f3-ac17-383678f0d713: TimelockConfig could be raised to u64::MAX immediately, making later reductions practically unrecoverable.
• A-092 / Nexus 98dbd8f7-6f6b-4096-b917-c01f9fd68e35: load_timelocks lazily materialized DataKey::Timelocks from the legacy scalar fallback during getter/view-style paths.

This PR keeps the timelock-model fixes consolidated in the existing governance timelock PR: it caps Soroban governance timelocks at 30 days, matching the NEAR vault governance upper bound, and makes legacy/fallback timelock reads side-effect-free while preserving explicit SetTimelock write behavior.

Changes

• Replace Soroban governance MAX_TIMELOCK_NS = u64::MAX with 30 * DAY_NS.
• Add regression test proving submit_set_timelock(..., TimelockKind::TimelockConfig, u64::MAX) returns GovernanceError::TimelockOutOfBounds and leaves the current TimelockConfig plus pending queue unchanged.
• Make load_timelocks return Timelocks::from_default(default_ns) without writing DataKey::Timelocks when only the legacy DataKey::TimelockNs scalar exists.
• Add regression test proving timelock_ns and timelocks getters do not materialize missing Timelocks storage.

Verification

Folded A-092 commit: 0358c07b9340ad820092dc8918a79238b8a241ec.

• cargo test -p templar-soroban-governance timelock_config_rejects_u64_max_without_mutating_state -- --nocapture
• cargo test -p templar-soroban-governance timelock_getters_do_not_materialize_missing_timelocks_storage -- --nocapture
• cargo test -p templar-soroban-governance -- --nocapture (31 passed)
• git diff --check origin/audit/governance-a040..HEAD
• just -f contract/vault/soroban/justfile size-budget-check

RED evidence recorded locally before fixes:

• A-040: focused regression failed with left: Ok(1), proving u64::MAX was accepted immediately.
• A-092: focused getter regression failed because timelock_ns wrote DataKey::Timelocks storage while reading from fallback state.

Runtime deploy WASM size from current folded branch size gate:

• 93961 bytes / 91.76 KiB
• Budget: 131072 bytes / 128 KiB

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[carrion256]

fixed on #457