Bump capnp-cpp past AnyStruct schema change and fix compatibility-date

[harrishancock]

capnproto/capnproto#2501 introduced a source-breaking change: schema::Value::Reader::getStruct() now returns capnp::AnyStruct::Reader (with as()) instead of capnp::AnyPointer::Reader (with getAs()).

Bump capnp-cpp past it and update the two getStruct().getAs() callers in compatibility-date.{c++,-test.c++} to use as().

Assisted-by: OpenCode:claude-opus-4.7

[ask-bonk]

No remaining callers of the old API. The change is complete and correct.

Let me verify the strip_prefix matches the new commit hash.

The old strip_prefix was capnproto-capnproto-23bb5b9 (first 7 chars of 23bb5b957cb1ab9f92734341b5664150e71890f2). The new strip_prefix is capnproto-capnproto-911e53d (first 7 chars of 911e53d67841687afe9a349fd8c0d39fe024515a). Both match correctly.

The change is straightforward and mechanically correct:

1. Dependency bump: capnp-cpp updated from 23bb5b9 to 911e53d, advancing 5 commits. SHA256 hash updated accordingly. strip_prefix matches the new commit.
2. API migration: Both callers of .getStruct().getAs<T>() are updated to .getStruct().as<T>(), matching the upstream API change from Use more precise AnyList and AnyStruct in the Value schema capnproto/capnproto#2501.
3. No missed callers: grep confirms no remaining uses of the old getStruct().getAs<>() pattern in the codebase.

LGTM

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