Remove leading debounces

[tudddorrr]

Trailing-only debounce

• Debounce behaviour changed from leading-and-trailing to trailing-only: the first call no longer fires immediately; all calls are deferred until the debounce window closes and then coalesced into a single API call.

Shared task result

• All callers within the same debounce window now share the same Task result instead of each receiving an independent TaskCompletionSource.

Simplified debounce internals

• Removed the SettleLeading method and the leading-call branch from Debounce(), leaving a single code path that always queues a trailing call.
• ProcessPendingUpdates no longer distinguishes between leading-in-flight and trailing-queued states; it simply re-opens the window if an execution is still running.

Test alignment

• Tests renamed and restructured to reflect trailing-only semantics (e.g. LeadingCall_* → TrailingCall_*).
• Assertions reordered so yield return null precedes assertions, giving the trailing call a frame to execute.
• debounceTimerSeconds set to 0f in test setup to make trailing calls fire immediately in the next frame.