fix(mechanism): make set_constraints change the dimensions the solver maintains

[HugoFara]

Found while wiring the editor's optimization endpoint onto Mechanism (pylinkage-editor).

Bug

Mechanism.set_constraints() did not change the mechanism. Link lengths live in Link._cached_distances and in the Assur-group solver's Dimensions, both captured at construction. set_constraints only moved the driver's output joint, so the next step() re-solved every joint against the original lengths — and put the crank tip back at its original radius, since DriverLink.radius reads the cache. get_constraints() (position-based for binary links) then read the old values too.

Reproduce on 1.2.1:

m = mechanism_from_dict(four_bar)          # constraints [70, 25, 60]
m.set_constraints([84.0, 30.0, 72.0])
next(m.step(1, dt=0.0))
m.get_constraints()                        # → [70.0, 25.0, 60.0]

Consequence: every optimizer run on a Mechanism (documented as "used for optimization" since 0.8.0) evaluated the same geometry for every particle.

Fix

• Link.set_distance(j1, j2, d) — writes the constraint (both orderings); positions are untouched and re-solved at the next step. Rejects non-positive distances.
• Mechanism.set_constraints — writes driver radii and binary link lengths through set_distance, repositions the crank tip at its current angle as before, and rebuilds the Assur decomposition so the group solver's Dimensions follow (0.02 ms on a four-bar vs ~10 ms per cycle simulation). Skips exactly the links get_constraints skips, so the two stay aligned.
• Link.length reads get_distance() (the maintained constraint once built), consistent with DriverLink.radius, so get/set_constraints round-trip before any step.

Scope unchanged: only binary links and driver radii form the constraint vector; n-ary links are still left out, as before.

Tests

TestGetSetConstraints: round trip before stepping; both solver paths (group_solver True/False) maintain the new lengths over 20 steps; to_dict/from_dict after a dt=0 step keeps them. TestLinkDistances: set_distance symmetry, positions untouched, validation. Full suite 2614 passed; ruff, mypy clean.