feat: Allow layer behaviors to "lock" layers on

[Nick-Munnich]

Adds a locking property to momentary, to, and toggle layer behaviors. Enabled by default for &to and &tog, a layer being "locked" prevents behaviors without the locking property from deactivating the layer.

Conditional layer activation and deactivation is always non-locking, which seems to interact nicely.

Common use case: with this PR you can place e.g. a &tog_on 1 or a &to 1 on layer 1, allowing you to "lock" it in place while it is active.

PR check-list

• Branch has a clean commit history
• Additional tests are included, if changing behaviors/core code that is testable.
• Pre-commit used to check formatting of files, commit messages, etc.
• Has any necessary documentation updates.

[caksoylar]

Conditional layer activation and deactivation is always non-locking, which seems to interact nicely.

If you want to get the state of a conditionally activated then-layer, it seems useful to me to propagate the locked state from if-layers (i.e. it is locked if all of the if-layers are locked). This'd be necessary to replace the momentariness tracking I mentioned on Discord.

Can you lock on a then-layer using &tog? I would guess not, since it still needs all the if-layers to be active.

[Nick-Munnich]

If you want to get the state of a conditionally activated then-layer, it seems useful to me to propagate the locked state from if-layers (i.e. it is locked if all of the if-layers are locked). This'd be necessary to replace the momentariness tracking I mentioned on Discord.

Can you lock on a then-layer using &tog? I would guess not, since it still needs all the if-layers to be active.

I realised that tracking that is unnecessary for this to work as expected. Rather, if both if-layers are locked then the then-layer is semi-locked: If it is deactivated, then on the event triggered by the deactivation the then-layer immediately becomes reactivated.

This also has the effect of allowing the then-layer to be locked using &tog, as if one of the if-layers becomes inactive then the conditional layer effect will try but fail to deactivate the then-layer.

[caksoylar]

I will rewind a bit: I have this momentariness tracking feature which is being used to filter layer change events that aren't "permanent". I am hoping to replace the tracking of momentariness by checking the "locked" state of the highest active layer instead. For that to work:

• There'd need to be a function like bool zmk_keymap_layer_locked(zmk_keymap_layer_id_t layer) that'd return the state of a layer
• Conditional layers would need to set the default locked status (check if all the if-layers are locked), before it might be locked directly by e.g. a &tog

[Nick-Munnich]

I am hoping to replace the tracking of momentariness by checking the "locked" state of the highest active layer instead.

I've added what I believe are the necessary parts to get that to work with the locking system.