codegen: add schema-evolution accessors

[nullstyle]

Summary

• add forward-compatible EnumOrdinals views, enum-list ordinal access, and whichOrdinal() while preserving exhaustive typed APIs and their existing error behavior
• add structural pointer-presence accessors plus Stable builder inspection primitives, and encode explicitly created empty structs with the reference-compatible offset -1 pointer
• add shared-ID V1/V2 fixtures, strict-validation regression coverage, deterministic generated-artifact checks, updated snapshots, docs, and Unreleased notes

This intentionally does not bump the version, tag, or publish a release.

Validation

• zig build test --summary all — 1,501/1,501 tests
• zig build test -Doptimize=ReleaseSafe --summary all — 672/672 tests
• focused codegen, message, schema-evolution, schema-validation, RPC-wire, serialization, interop, WASM, and docs suites
• zig build check-test-compile -Dtarget=x86_64-windows --summary all — 87/87 test binaries
• Stable API snapshot and closure in default and QUIC roots
• just check-generated, just fmt-check, and git diff --check