Fix sentencepiece strict hdrs_check failure when building //c:litert-lm on macOS

[yudaprama]

Problem

Building the C API shared library on macOS (Apple Silicon) fails during compilation of the sentencepiece dependency:

bazel build //c:litert-lm --config=macos_arm64

ERROR: external/sentencepiece/BUILD.bazel:23:11: Compiling src/word_model.cc failed: undeclared inclusion(s) in rule '@@sentencepiece//:sentencepiece_processor':
this rule is missing dependency declarations for the following files included by 'src/word_model.cc':
  'bazel-out/darwin_arm64-opt/bin/external/sentencepiece/sentencepiece_model.pb.h'

With the current http_archive stanza (v0.2.2, no strip beyond the default), the generated sentencepiece_model.pb.h lands in the package root of the external repo while the processor sources live under src/, so the quoted include resolves through the includes = ["src"] path instead of through the declaring cc_proto_library label — and strict hdrs_check rejects it. The same applies to //support/tokenizer:sentencepiece_tokenizer.

Fix

Adopt a strip-to-src layout for sentencepiece v0.2.2:

• WORKSPACE: strip_prefix = "sentencepiece-0.2.2/src" so sources and the generated .pb.h share the package root (quoted includes, no cross-dir resolution), plus sed of third_party/absl/ and third_party/protobuf-lite/ include prefixes to the real absl/protobuf.
• PATCH.sentencepiece_darts: re-adds the vendored third_party/darts_clone/darts.h that the strip drops — v0.2.2 sources require its newer DoubleArrayImpl API (copy_array, validate), which the older separately-pinned darts_clone does not have.
• BUILD.sentencepiece: root-file layout, third_party/darts_clone/darts.h in hdrs, and the full absl dep set v0.2.2 sources actually include.

Verification

bazel build //c:litert-lm --config=macos_arm64 --jobs=6
INFO: Build completed successfully

macOS 15, Apple Silicon (darwin_arm64), Bazel 7.6.1 via bazelisk. Linking the resulting liblitert-lm.dylib from a Rust FFI consumer and running a Gemma 4 E2B (.litertlm) streaming session works end to end.