Cache undefined widening properties per symbol, not per name

[Max Schwenk (maschwenk)]

Fixes #64727

getUndefinedProperty cached the synthetic prop?: undefined symbol by name, so the first object literal to widen a name decided how that property printed in later, unrelated types. declaration output then depended on which other files shared a checker, and on --checkers.

this keys the cache by the source property symbol instead. it's a 4 line change in checker.go.

• new test undefinedPropertyDeclarationOrder covers the a.ts/b.ts repro
• objectLiteralNormalization baselines reorder: c3, c4 and e2 now match 6.0.3 exactly, nothing moves further from it
• emitted b.d.ts is byte-identical with and without a.ts at --checkers 1, 2, 4 and 8. before the fix it differed at 1, 2 and 4
• hereby validate passes except test:extension, which fails the same way on clean main

ai disclosure: written with claude code. i've read the change and will handle review myself.