Closes #2600

This adds an on-output match property to window rules, so you can apply different rules to windows depending on which monitor they're on. It matches by connector name, like "DP-1", or make/model/serial, same style as output config blocks.

Motivation

I use a Framework 13 laptop with a 32" external monitor and want different window sizing per display.

A specific example:

• I want to open a Firefox window at 1/3 width on the big 32" monitor, but maximized on other screens.
• I want messaging clients (discord, signal, etc) to open at 1/2 width on the 32" monitor, but maximized on other screens.

Per-output layout blocks get you part of the way there but they apply to all windows on that output, not specific ones.

Here's a snippet of what my config.kdl looks like now with these things applied:

// Firefox: 1/3 width on the MSI 32"
window-rule {
    match app-id="firefox" on-output="Microstep MSI MAG321CQR KA3H071804955"
    default-column-width { proportion 0.33333; }
}

// Firefox: open maximized on all other displays
window-rule {
    match app-id="firefox"
    exclude on-output="Microstep MSI MAG321CQR KA3H071804955"
    open-maximized true
}

// Messaging clients: 1/2 width on the MSI 32"
window-rule {
    match app-id=r#"^vesktop$"# on-output="Microstep MSI MAG321CQR KA3H071804955"
    match app-id=r#"^Element"# on-output="Microstep MSI MAG321CQR KA3H071804955"
    match app-id=r#"^signal$"# on-output="Microstep MSI MAG321CQR KA3H071804955"
    default-column-width { proportion 0.5; }
}

// Messaging clients: open maximized on all other displays
window-rule {
    match app-id=r#"^vesktop$"#
    match app-id=r#"^Element"#
    match app-id=r#"^signal$"#
    exclude on-output="Microstep MSI MAG321CQR KA3H071804955"
    open-maximized true
}

Notes

Circular dependency guard

Like discussed in #2600, on-output and open-on-output in the same rule would create a circular dependency so we'd need to know the target output to evaluate the rule, but the rule changes the target output. So this is handled with:

• A config-level validation that rejects any window rule combining on-output matchers with open-on-output or open-on-workspace
• A two-pass evaluation at window open time. The first pass resolves open-on-output/open-on-workspace redirects without output context, second pass re-evaluates with the resolved target output for on-output matching

I think this implementation is sensible, I would appreciate any feedback / help with this here!

Changes

• niri-config: add on_output field to Match, reject open-on-output/open-on-workspace in rules with on-output
• src/window: thread Option<&OutputName> through ResolvedWindowRules::compute and window_matches
• src/handlers/xdg_shell.rs: two-pass rule evaluation in send_initial_configure, preliminary pass for output/workspace redirects, then full pass with resolved output
• src/niri.rs: pass output context when recomputing rules for mapped/unmapped windows
• Wiki: document on-output with examples and must-fail block for the circular case