[fontdrasil] Fix many-to-one axis map handling

[anthrotype]

This is the companion to fonttools/fonttools#4085.

Fixes the remaining ttx_diff discrepancy on SUSE-Italic at 7159afb255 reported in googlefonts/ufo2ft#978

"output is identical" in ttx-diff after updated fonttools is installed in the fontc .venv.

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PiecewiseLinearMap::map() uses binary_search, which returns an arbitrary/unspecified index when there are multiple matches:

https://doc.rust-lang.org/std/primitive.slice.html#method.binary_search

When a many-to-one forward map (e.g. user=900 -> design=1000 and user=1000 -> design=1000) is reversed, the value returned from binary_search can differ from fontmake's (which takes the first match).

I changed it to use partition_point to always find the first occurrence.

After fixing this I found a second related issue: to_ir_axis derived fvar axis min/default/max by round-tripping design coords through design_to_user, which is lossy for many-to-one maps.

Changed it to take min/max directly from the user-space values in the mapping, matching glyphsLib's approach.

(I will release a patched fonttools soon and bump fontc requirements after)

[anthrotype]

https://github.com/googlefonts/fontc/actions/runs/25220991765/job/73952882058?pr=1995#step:4:195

thread 'piecewise_linear_map::tests::many_to_one_reverse' (3210) panicked at fontdrasil/src/piecewise_linear_map.rs:151:9:
assertion `left == right` failed
  left: 1000.0
 right: 900.0

[cmyr]

nice!