Missing `stack-extend` and `stack-increase`

[jannik-stacks]

Missing stack-extend and stack-increase — solo stackers can't renew or top up

Potentially optional, but I'm tracking it here anyway for future notes.

Right now, the SDK only supports stack-stx for solo stacking. Once your STX are locked, you're stuck — there's no way to extend the lock period or increase the locked amount without waiting for unlock.

What's missing

Contract function	What it does	Upstream stacks.js
stack-extend	Extend an existing solo stack by N more cycles	StackingClient.stackExtend()
stack-increase	Add more STX to an existing solo stack position	StackingClient.stackIncrease()

Why this matters

A solo stacker who wants to keep stacking across multiple cycles has to:

1. Wait for their current lock to expire
2. Call stack-stx again from scratch

That gap between unlock and re-stack means missed rewards for at least one cycle. stack-extend exists specifically to avoid this — you call it before your lock expires and seamlessly continue into the next cycles.

Similarly, if a stacker acquires more STX and wants to increase their stacking position, they currently can't. They'd have to wait for unlock and re-stack the full amount.

[saraidy]

Working on implementing those two additional functions in the SDK.