…nhacks#2854, colinhacks#2106)

`.deepPartial()` was deprecated in v3.21.0 and removed in v4 because the
underlying recursive `instanceof` traversal had structural problems
(see colinhacks#2106): it could not see user-defined schema subclasses, accumulated
edge cases for every new schema type, and silently broke on advanced
shapes. The community has been asking for a working alternative ever
since (colinhacks#2854, 60+ commenters).

Adds a v4-native implementation that addresses the design concerns of
colinhacks#2106 head-on:

- Dispatches on `schema._zod.def.type`, not `instanceof`. Custom schemas
  with unknown `def.type` fall through to identity instead of being
  silently mishandled. Users intercept inside `fn` for custom rewriting.
- Cycle-safe via a `Map<schema, RESOLVING|result>` cache. Lazy schemas
  unfold lazily so recursive shapes terminate; shared sub-schemas are
  visited once. Direct (non-`lazy`) cycles throw rather than loop.
- Bottom-up rewrite as a primitive. `core.mapOnSchema(schema, fn)` is
  the general traversal; `deepPartial` is one line on top of it.
  `deepStrict`, `deepReadonly`, etc. become trivial follow-ups.
- Covers the v4 def vocabulary: object, array, tuple (+rest), record,
  map, set, union (incl. discriminated union — discriminator metadata
  preserved by spreading `def`), intersection, optional, nullable,
  default, prefault, nonoptional, catch, readonly, promise, success,
  pipe, function, lazy. Leaves (primitives, enum, literal, transform,
  custom, file, etc.) returned untouched.

`mapOnSchema` lives in `core/` so both `classic` and `mini` reuse it;
each variant has its own thin `deepPartial` wired to its own `partial`.

Traversal pattern adapted from Jaen's v3 `mapOnSchema` (Apache-2.0,
https://gist.github.com/jaens/7e15ae1984bb338c86eb5e452dee3010).
Implementation rewritten for v4: `def.type` dispatch instead of
`instanceof`, schema construction via `core.clone` instead of
`new ZodFoo(...)`, full v4 def coverage.

```ts
const Node: z.ZodType = z.object({
  name: z.string(),
  children: z.array(z.lazy(() => Node)).optional(),
});

z.deepPartial(Node).parse({});                    // ok
z.deepPartial(Node).parse({ children: [{}] });    // ok
```

Adds 22 tests covering shallow + nested objects, arrays, tuples with
rest, unions, optional/nullable wrappers, recursive (lazy) schemas,
primitive leaf preservation, and `mapOnSchema` identity / targeted
rewrites.

CI on TS 6+ may surface the unrelated `baseUrl` deprecation error
from colinhacks#5921 until that lands; this PR doesn't touch that path.

…nhacks#2854, colinhacks#2106)

`.deepPartial()` was deprecated in v3.21.0 and removed in v4 because the
underlying recursive `instanceof` traversal had structural problems
(see colinhacks#2106): it could not see user-defined schema subclasses, accumulated
edge cases for every new schema type, and silently broke on advanced
shapes. The community has been asking for a working alternative ever
since (colinhacks#2854, 60+ commenters).

Adds a v4-native implementation that addresses the design concerns of
colinhacks#2106 head-on:

- Dispatches on `schema._zod.def.type`, not `instanceof`. Custom schemas
  with unknown `def.type` fall through to identity instead of being
  silently mishandled. Users intercept inside `fn` for custom rewriting.
- Cycle-safe via a `Map<schema, RESOLVING|result>` cache. Lazy schemas
  unfold lazily so recursive shapes terminate; shared sub-schemas are
  visited once. Direct (non-`lazy`) cycles throw rather than loop.
- Bottom-up rewrite as a primitive. `core.mapOnSchema(schema, fn)` is
  the general traversal; `deepPartial` is one line on top of it.
  `deepStrict`, `deepReadonly`, etc. become trivial follow-ups.
- Covers the v4 def vocabulary: object, array, tuple (+rest), record,
  map, set, union (incl. discriminated union — discriminator metadata
  preserved by spreading `def`), intersection, optional, nullable,
  default, prefault, nonoptional, catch, readonly, promise, success,
  pipe, function, lazy. Leaves (primitives, enum, literal, transform,
  custom, file, etc.) returned untouched.

`mapOnSchema` lives in `core/` so both `classic` and `mini` reuse it;
each variant has its own thin `deepPartial` wired to its own `partial`.

Traversal pattern adapted from Jaen's v3 `mapOnSchema` (Apache-2.0,
https://gist.github.com/jaens/7e15ae1984bb338c86eb5e452dee3010).
Implementation rewritten for v4: `def.type` dispatch instead of
`instanceof`, schema construction via `core.clone` instead of
`new ZodFoo(...)`, full v4 def coverage.

```ts
const Node: z.ZodType = z.object({
  name: z.string(),
  children: z.array(z.lazy(() => Node)).optional(),
});

z.deepPartial(Node).parse({});                    // ok
z.deepPartial(Node).parse({ children: [{}] });    // ok
```

Adds 22 tests covering shallow + nested objects, arrays, tuples with
rest, unions, optional/nullable wrappers, recursive (lazy) schemas,
primitive leaf preservation, and `mapOnSchema` identity / targeted
rewrites.

CI on TS 6+ may surface the unrelated `baseUrl` deprecation error
from colinhacks#5921 until that lands; this PR doesn't touch that path.