[belisoful]

Part of #1497. Found by the second Collections path audit.

• Read-only TWeakMap: clear() had no read-only check, so clear() and copyFrom() emptied a read-only map, and copyFrom(['b' => 2]) emptied it and then threw. It now throws map_readonly like TMap and leaves the map unchanged.
• Explicit WeakReference: TWeakList and TWeakMap tracked the reference, which the collection holds strongly, so a collected referent was never scrubbed. A reference to a TEventHandler also drove TWeakList's handler count to -1 on removal. Add and remove now track the referent.
• Reentrant clear(): all three collections crashed when a destructor removed another entry during the clear. Each now re-reads its live state on every pass and ends empty.
• Read-only TWeakCallableCollection: turning DiscardInvalid on threw list_readonly, against its documentation. It now removes collected entries as TWeakList and TWeakMap do.
• Stale counts: clear() resets the handler count, and removing a dead handler with DiscardInvalid false now decrements it.
• Failed inserts: TWeakList and TWeakCallableCollection track an item after the insert succeeds, so a failed insert leaves no bookkeeping.

Behavior to review:

• A read-only TWeakList with DiscardInvalid on that holds only collected entries now clears without error, as TWeakMap does; it scrubs first and then has nothing to remove. It used to throw list_readonly.
• A WeakReference to a TEventHandler is treated as a plain weak object: the handler itself is tracked, and it is not counted as an event handler.

Not changed: if a destructor calls remove() on an entry clear() has already removed, TWeakList and TWeakCallableCollection still throw list_item_inexistent out of clear(), because swallowing it would hide real errors. The trait docblock records this. detachEventHandler() catches it and returns false.

26 regression tests, passing with and without Xdebug develop mode. Those tests and #1498 pass together.