Fix HookOn v2 direction-only updates

[sublimator]

Summary:

• validate HookOn v2 direction-only fields in the NOOP path
• avoid dereferencing an empty defHookOn optional during updates
• document why defHookOn may be empty for direction-only definitions

Tests:

• x-run-tests --reconfigure-build --stop-on-fail -- SetHook7
• x-run-tests --no-build --stop-on-fail -- SetHook

[sublimator]

This follow-up should address the current Xahau PR Xahau#766 SetHook failures.

Summary:

• fixes the HookOn v2 direction-only defHookOn empty optional dereference in update merge
• rejects lone HookOnIncoming / HookOnOutgoing fields that were previously inferred as NOOP under fixHookOnV2InstallUpdate
• adds direct regression coverage for both lone-direction NOOP shapes

Local verification on f5c5270d55e1dcbd3fbe6c77e8e91dc0b2c1503b:

• x-run-tests --stop-on-fail -- SetHook7
• x-run-tests --no-build --stop-on-fail -- SetHook

Two fresh review passes found no code/consensus blockers. gh pr checks reports no checks for this fork PR, so the practical next step is to merge this into fixHookOnV2InstallUpdate and let Xahau#766 CI rerun.