fix: make default JSON serializer contract explicit

[lan17]

Summary

Make DialCache's default JSON serializer contract honest without adding a validation pass or changing runtime serialization behavior.

The default serializer now advertises unknown rather than allowing callers to claim arbitrary lossless T round trips. The README documents the exact JSON-compatible domain, representative lossy transformations, native failures, and the custom serializer escape hatch.

Architecture and design

• Keep JSON.stringify/JSON.parse as the default wire codec.
• Treat Serializer<T> as a trusted codec boundary: DialCache does not perform an additional serialize/deserialize validation round trip.
• Preserve the existing top-level undefined sentinel.
• Preserve fail-open behavior for actual serializer exceptions.
• Leave silent native JSON transformations as an explicit caller precondition rather than adding per-cache-write CPU overhead.

API impact

• JsonSerializer is no longer generic; it implements Serializer<unknown>.
• JsonSerializer.load() returns Promise<unknown>.
• Existing code using new JsonSerializer<MyType>() must use new JsonSerializer() and narrow the decoded value, or provide a typed custom Serializer<MyType>.
• The custom Serializer<T> interface remains available and unchanged for types such as Date.

There are no Redis key, frame, Lua, adapter, or stored-payload format changes.

Operational behavior

This adds no runtime validation and no extra serialization pass, so the cache hot path has no new CPU overhead. Existing JSON-compatible cached values remain wire-compatible.

The documented default domain includes JSON-stable primitives, arrays, and plain objects. Values such as Date, Map, Set, class instances, non-finite numbers, nested undefined, symbols, functions, bigint, cycles, prototypes, and shared object identity may be rejected or changed by native JSON semantics.

Validation

• corepack pnpm typecheck
• Unit and coverage suite: 103 tests passed
• Focused serializer and Redis tests: 20 tests passed
• Release tests: 5 tests passed
• Build and declaration emission
• Packed-package consumer test on Node 20.17.0
• Integration suite: 46 tests passed
• git diff --check

Tracking

Closes #16