[ty] Fix panic from mismatched OR-pattern bindings (#27533)
## Summary
https://github.com/astral-sh/ruff/pull/27438 modeled OR-pattern
alternatives as distinct control-flow paths rather than linear control
flow. This is correct, but the implementation was inadequate; it
effectively treated all of these branches simultaneously as "definitely
taken", which becomes a problem in the (invalid syntax) case where not
all branches bind the same name(s). That caused a panic.
Fix this by introducing reachability predicates to explicitly model that
only one of these "branches" can be taken.
Closes astral-sh/ty#4194.
## Test plan
Added mdtests covering distinct and overlapping captures, alternatives
without bindings in either order, three-way alternatives, previously
bound names, nested mismatches, guard references, and the original
malformed-case panic.
