Move the legacy buffered-stream dispatcher out of the C decompression adapter
while keeping ZSTD_DCtx_s private and C-owned. Rust now consumes the existing
ZSTD_rustDctxView projection, detects the legacy version at input.pos, preserves
static-context rejection and stream-stage reset behavior, and handles legacy
context replacement through the version-gated v0.4-v0.7 Rust stream APIs.
The Rust boundary retains the C helper's NULL normalization, dictionary
lifetime sentinel, previous-version cleanup semantics, input/output position
updates, and version-unsupported and no-legacy-support fallbacks. The C legacy
free helper remains unchanged; only the dispatcher declaration and body were
removed from the adapter. Focused tests cover unsupported versions, stage
reset, context reuse/switching, and partial v0.4 input/output progress.
Test Plan:
- `cargo clippy --manifest-path rust/Cargo.toml --no-default-features --features compression` -- passed
- Same clippy command with `--benches` and `--tests` -- passed
- `cargo +nightly fmt --manifest-path rust/Cargo.toml`, followed by all three clippy commands -- passed
- Decompression/legacy clippy with `-D warnings` -- passed
- All 107 Rust tests with v0.4-v0.7 features -- passed
- Focused legacy dispatcher tests: 4 passed
- `make -C lib -j2 lib-mt` and `make -C lib -j2 lib-nomt` -- passed
- `make -C tests -j2 test-legacy` -- built, then failed in the existing simple one-shot path with one v0.8 output byte mismatch (`e0` vs expected `e2`) before the streaming check; the new dispatcher symbol was not called on that failing path
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