refactor(protoc): migrate to protocompile experimental compiler with embedded proto deps

[Kybxd]

Why

Protobuf Edition 2024 is not understood by bufbuild/protocompile's
stable Compiler. To unblock Edition 2024 schemas in this codebase, we
follow the same path Buf itself took in
buf v1.69.0 and
switch the internal protoc wrapper to protocompile's experimental
compiler, which has Edition 2024 support.

What changes vs. the stable compiler

The experimental compiler exclusively consumes source files via
source.Opener. It no longer accepts pre-compiled FileDescriptors
through protoregistry.GlobalFiles — which is how the stable compiler
used to pick up tableau/protobuf/*.proto and
buf/validate/validate.proto. We need a new mechanism to make those
imports resolvable at compile time.

How

Vendor the required .proto files under internal/x/xproto/protoc/embedded/
and expose them via go:embed:

Embedded file	Source
tableau/protobuf/tableau.proto	copied from proto/tableau/protobuf/
tableau/protobuf/wellknown.proto	copied from proto/tableau/protobuf/
buf/validate/validate.proto	buf export of buf.build/bufbuild/protovalidate at the commit pinned in buf.lock

Standard imports (google/protobuf/*) continue to be served by
source.WKTs and need no embedding.

Refresh workflow

A go:generate directive in embed.go invokes gen_embedded.sh, which:

1. Wipes embedded/ and recreates embedded/tableau/protobuf/.
2. Copies proto/tableau/protobuf/*.proto (top-level only — matches
   buf.yaml excludes for internal/ and unittest/).
3. Reads the protovalidate commit out of buf.lock and forwards it to
   buf export buf.build/bufbuild/protovalidate:<commit>.

⚠️ buf export <remote-module> does not consult the workspace's
buf.lock — that file only governs intra-workspace transitive
resolution. Without an explicit commit suffix, buf export would pull
the latest published commit. The script reads the commit from
buf.lock so the vendored copy stays in sync automatically.

Why is embedded/ checked into version control?

So that downstream consumers of
go install github.com/tableauio/tableau/cmd/tableauc@latest can build
without buf installed and without running go generate —
proxy.golang.org only packages git-tracked files.

Upgrading protovalidate

buf dep update          # bumps buf.lock
go generate ./...       # refreshes embedded/
git add -A && git commit

[codecov]

Codecov Report

❌ Patch coverage is 60.24096% with 33 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.44%. Comparing base (99d72fc) to head (227b52c).

Files with missing lines	Patch %	Lines
internal/x/xproto/protoc/compiler.go	58.46%	18 Missing and 9 partials ⚠️
internal/protogen/protogen.go	66.66%	2 Missing and 2 partials ⚠️
internal/x/xproto/protoc/embed.go	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #404      +/-   ##
==========================================
+ Coverage   73.38%   74.44%   +1.05%     
==========================================
  Files          88       89       +1     
  Lines       11300     9154    -2146     
==========================================
- Hits         8293     6815    -1478     
+ Misses       2440     1762     -678     
- Partials      567      577      +10     

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