BA availability and sqd# size mismatch results in no faction availability for many sqd# sizes

There are numerous BA sqd#s that are effectively orphaned by not having availability adjudicated separately from squad size. It's an understandable way to do things, especially for procedurally generated forces to have generally appropriate squad sizes. However, I as a player should have the option of fielding under/over strength squads. For example, a clan force could underbid a point of BA, or a mercenary force might field whatever number they have working, or a house force may field a reinforced squad for a mission.

Specific example

1. Elemental BA (laser) (or really any of the original Elemental configs) were captured in numbers during various fights (and presumably various understrength points fought as well). However the availability of sqd4 doesn't begin until the Republic Era and sqd 3 only exists for the Society and only during the Jihad era. Sqd6 is never available at all. I should also note that all sqd3 BA are a result of Society, so there are only a handful of types available in sqd3 at all.

2. The Corona (SRM) is an interesting case since the sqd5 is the only one that actually exists, but MM has the procedurally generated sqd4 and sqd6 with no availability ever, Even the sqd5 only exists for for Clan Star Adder and only during the Jihad.

Ideally, I would be able to field under/overstrength squads of BA that are available to my faction. However, the current availability system hides or makes it inconvenient to select short/reinforced squads.

Start by examining MegaMek's availability adjudication for Battle Armor squad sizes, using the Elemental and Corona examples in the issue as cases. Determine how understrength and overstrength squads should inherit faction and era availability, then verify that affected squad sizes become selectable without exposing unavailable configurations.