refactor(build): vendor prost output, drop build.rs

[GordonYuanyc]

Summary

Switch the crate's protobuf code generation from a per-build script to a
vendored / checked-in model. Downstream users now get the prost-generated
Rust types directly in the crate; they no longer need protoc or any
[build-dependencies] to build asap_sketchlib.

What changes for downstream users

• cargo add asap_sketchlib no longer pulls prost-build or
  protoc-bin-vendored, nor any of their ~30 transitive deps.
• No build script runs on their machine; the crate is pure Rust.
• The public surface crate::proto::sketchlib::* is byte-equivalent to
  the previous build-time output (verified by the existing
  cross_language_proto test).

What changes for maintainers

• Source of truth is still proto/**/*.proto.

• After editing any .proto, regenerate the vendored output with:

  cargo run --manifest-path tools/gen-proto/Cargo.toml
  git add src/proto/generated

• A new CI job proto-vendored-up-to-date runs the generator and fails the
  PR if git diff --exit-code -- src/proto/generated shows drift.

File-level changes

• Deleted build.rs.
• New tools/gen-proto/ — standalone (publish = false, own
  [workspace]) maintainer tool that drives prost-build +
  protoc-bin-vendored and writes to src/proto/generated/. Because it has
  its own Cargo.toml, Cargo automatically excludes the directory from the
  published tarball.
• New src/proto/generated/sketchlib.v1.rs — the vendored prost output
  (652 lines).
• Modified src/proto.rs — include! now points at the vendored path;
  the rustfmt/clippy attributes moved from inner attributes inside the
  generated file (illegal in an include! expansion) to outer attributes on
  the wrapping pub mod sketchlib.
• Modified Cargo.toml — removed [build-dependencies], added an
  explanatory comment.
• Modified Cargo.lock — ~280 lines (~30 transitive crates) dropped.
• Modified .github/workflows/ci.yml — added the
  proto-vendored-up-to-date job.
• Modified .github/workflows/docs.yml — paths filter swaps
  build.rs for tools/gen-proto/**.
• Updated README.md and docs/library_map.md to describe the new
  generation workflow.

Test plan

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings
• cargo test --all-features --locked (including the
  cross_language_proto roundtrip test that validates wire-format
  compatibility with sketchlib-go)
• RUSTDOCFLAGS=-D warnings cargo doc --no-deps --all-features --locked
• cargo package --list --allow-dirty confirms tools/gen-proto/ is
  not shipped to crates.io and src/proto/generated/sketchlib.v1.rs is.
• cargo metadata --no-deps confirms tools/gen-proto is not a
  member of the asap_sketchlib workspace.
• CI green on PR — including the new drift-protection job.

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