Clean up and refactor the Unit class (#769)

## Description:

* Merged similar fields so they can be reused (eg warshipTarget =>
targetUnit)
* simplified isCooldown api
* added "touch" method to send update to UI layer
* standardized on "undefined"


## Please complete the following:

- [ ] I have added screenshots for all UI updates
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
- [x] I understand that submitting code with bugs that could have been
caught through manual testing blocks releases and new features for all
contributors

## Please put your Discord username so you can be contacted if a bug or
regression is found:

<DISCORD USERNAME>
evan

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- **Refactor**
- Unified and simplified how unit targets and cooldowns are managed
across all unit types, resulting in more consistent in-game behavior for
nukes, warships, trade ships, and SAM launchers.
- Updated naming and logic for unit targeting and cooldowns, improving
clarity in status displays and interactions.
- Reorganized unit interface and streamlined cooldown handling for
smoother gameplay experience.
- **Bug Fixes**
- Corrected visual indicators for nukes and warships to accurately
reflect their targets.
- **Tests**
- Updated automated tests to align with the new cooldown and targeting
logic.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

src/core/execution/SAMLauncherExecution.ts

@@ -135,10 +135,10 @@ export class SAMLauncherExecution implements Execution {
           unit.owner() !== this.player && !this.player.isFriendly(unit.owner()),
       )
       .filter((unit) => {
-        const dst = unit.detonationDst();
+        const dst = unit.targetTile();
         return (
           this.sam !== null &&
-          dst !== null &&
+          dst !== undefined &&
           this.mg.manhattanDist(dst, this.sam.tile()) <
             this.MIRVWarheadProtectionRadius
         );
@@ -149,19 +149,17 @@ export class SAMLauncherExecution implements Execution {
       target = this.getSingleTarget();
     }
 
-    if (
-      this.sam.isCooldown() &&
-      this.sam.ticksLeftInCooldown(this.mg.config().SAMCooldown()) === 0
-    ) {
-      this.sam.setCooldown(false);
+    if (this.sam.ticksLeftInCooldown() === 0) {
+      // Touch SAM to update sprite to show not in cooldown.
+      this.sam.touch();
     }
 
     const isSingleTarget = target && !target.targetedBySAM();
     if (
       (isSingleTarget || mirvWarheadTargets.length > 0) &&
-      !this.sam.isCooldown()
+      !this.sam.isInCooldown()
     ) {
-      this.sam.setCooldown(true);
+      this.sam.launch();
       const type =
         mirvWarheadTargets.length > 0 ? UnitType.MIRVWarhead : target?.type();
       if (type === undefined) throw new Error("Unknown unit type");