Add combining avagraha to Tiro Bangla?

[tiroj]

https://www.unicode.org/Public/UCD/latest/ucd/ScriptExtensions.tx
includes
A8F1 ; Beng Deva # Mn COMBINING DEVANAGARI SIGN AVAGRAHA
Should this character be added to the Tiro Bangla font and, if so, what form should it take? The reference form in Unicode is based on that of the Devanagari avagraha (U+093D), but the Bangla script uses a different avagraha character (U+09FE). Should the Bangla combining avagraha shape follow that of the Bangla avagraha?

See also:
https://twitter.com/TiroTypeworks/status/1561815478196006912

[tiroj]

Now testable with v160beta.yml build path.

[tiroj]

Note: this and the /bSandhi mark have inherited anchor position from the chandrabindu, but raised 200 units. This might not be ideal in all possible situations, so these marks might benefit from their own anchors, or be treated more like the Samaveda marks in the Tiro Devangari Sanskrit font, i.e. at a fixed height.