Improve discriminated union implementation

[colinhacks]

Remove the disc field from internals and replace with a simple propValues map. This makes introspection into discriminator values far easier, simplifies the parsing logic, and results in a 2x performance improvement:

$ cd packages/bench
$ pnpm bench discriminated-union
cpu: Apple M1 Max
runtime: node v22.13.0 (arm64-darwin)

benchmark           time (avg)             (min … max)       p75       p99      p999
------------------------------------------------------ -----------------------------
• z.disriminatedUnion().parse
------------------------------------------------------ -----------------------------
zod 3           33'996 ns/iter    (31'625 ns … 321 µs) 32'416 ns    129 µs    189 µs
zod 4 (after)    9'528 ns/iter     (9'000 ns … 223 µs)  9'209 ns 10'083 ns    117 µs
zod 4 (before)  19'161 ns/iter    (18'291 ns … 207 µs) 18'584 ns 32'542 ns    128 µs

summary for z.discriminatedUnion().parse
  zod 4 (after)
   2.01x faster than zod 4 (before)
   3.57x faster than zod 3