parakeet-cpp: address PR #22 AOSC v2.1 review comments

[pratiknarola-t]

Resolves the review comments on the merged AOSC v2.1 PR (#22, merge commit e6ba38c). All eight changes are minimal and behaviour-preserving except the v2.1 detection upgrade (now strict-tag with shape fallback) and the degenerate-config guard (silence-only fallback instead of UB-adjacent boost arithmetic). Reviewer comments classified as "perf only / out of scope / would only add a TODO" are intentionally not addressed in this commit -- see the plan file referenced in the PR description.

src/parakeet_sortformer.cpp -- compress_speaker_cache

• Early-return when spkcache_len_per_spk <= 0 (num_spks * A_sil >= spkcache_len). The downstream boost/top-K stages are mostly defended (boost_topk_scores already returns early on non-positive k), but the function was otherwise running a no-op pass that produced an all-silence cache via the slow path. Fall back to an explicit silence-only profile and bail.
• Renamed streaming_update's chunk_pre_encode_lc parameter to committed_chunk_pre_encode. The call site already advances past the left context (chunk_pre_committed = ... + lc * D), so the old _lc suffix was misleading. int lc stays -- it's used inside the function to index into preds_full, which still contains the left-context preds.
• Replaced the magic -1.0e30f / +1.0e30f sentinels (4 sites) with named constants k_score_neg_inf / k_score_pos_inf backed by std::numeric_limits<float>::{lowest,max}(). Dropped the inline "-inf is UB with current FP flags" comments: IEEE 754 +/-inf is well-defined; the original concern (avoiding NaN-on-arithmetic) still holds because we only store and compare the sentinels.

src/parakeet_engine.cpp

• On the AOSC path, skip the for (cur_full) remap_id(...) loop and the prev_chunk_full_segments = std::move(cur_full) store: compute_slot_remap_ is never consulted when cache_active is true (AOSC anchors slot identity through the speaker cache), so the work was dead.
• Switched v2.1 detection from pure-shape to "prefer the converter's parakeet.model_variant GGUF tag; fall back to (n_layers == 17, n_mels == 128) for legacy GGUFs". This prevents a future v2.2/v3 variant that happens to share v2.1's encoder shape from silently opting into AOSC.

include/parakeet/diarization.h

• Moved the v1-vs-v2.1 detection rationale comment out of parakeet_engine.cpp and into the SortformerStreamingOptions:: spkcache_enable block, with a paragraph on the tag-first / shape-fallback policy.

src/parakeet_ctc.{h,cpp}

• Added std::string ParakeetCtcModel::model_variant (optional GGUF metadata mirror; empty on legacy GGUFs).
• Loader reads parakeet.model_variant next to the existing parakeet.model.type read; absent key -> empty string -> detection falls back to shape.

scripts/convert-nemo-to-gguf.py

• New detect_sortformer_variant(ckpt: Path) derives a stable variant tag from the source .nemo filename (sortformer-v1 / sortformer-streaming-v2 / sortformer-streaming-v2.1-aosc); empty string for unknown checkpoints.
• Sortformer branch of write_gguf writes parakeet.model_variant when the tag is non-empty.
• write_gguf signature extended with ckpt: Path; only the one internal call site adjusted.

scripts/download-all-models.sh

• Added the diar_streaming_sortformer_4spk-v2.1 fetch block (the AOSC fine-tune that this PR's tests target); bumped the budget comment from "~14 GiB" to "~14.5 GiB" and listed v2.1 in the contents line.

CMakeLists.txt + test/test_sortformer_streaming.cpp

• Streaming ctest now consumes ${_qvp_sfsv21_q8_gguf} (was ${_qvp_sfs_q8_gguf}, the v2 model). The in-binary default GGUF path is the matching v2.1 q8_0. Aligns the test with the line-299 comment that says the binary "reflects the production v2.1 AOSC config out of the box".

test/test_utils.h (new) + test/test_sortformer_{streaming,aosc_speakers}.cpp

• Extracted the two 40-line load_wav_pcm16le_mono / file_exists duplicates into a shared inline header in the parakeet_test namespace. The duplicate copies and the "duplicated here on purpose" comment block in test_sortformer_aosc_speakers.cpp are gone; both tests #include "test_utils.h" and use using parakeet_test::....

Build + ctest verification

• cmake --build build -j clean (no new warnings).
• ctest -R 'test-sortformer-(streaming |aosc-speakers)':
  test-sortformer-streaming ........ Passed 8.23 s
  test-sortformer-aosc-speakers-abcba . Passed 33.80 s
  test-sortformer-aosc-speakers-abcdba Passed 36.91 s
  The locally-symlinked v2.1 GGUF predates the parakeet.model_variant
  key, so the AOSC tests passing here also verifies the shape-fallback
  path. Re-running the converter on the v2.1 .nemo will populate
  the new key for the strict-tag path.

Reviewer comments deferred / skipped (rationale):

• Encoder graph cache thrashing during FIFO ramp-up (number 4): perf only; proper fix wants pre-build-at-diarize_start + silence padding or a mask argument, not minimal. Tracked for a follow-up perf PR.
• ring.erase O(n) under AOSC's aggressive trim (number 10): pre-existing on the v1 path; wants a std::deque refactor, out of scope.
• encoder_ms attribution surprising (number 12): code is correct and matches sibling paths; the user explicitly opted against comment-only "clarifications".

[pratiknarola-t]

The Encoder graph caching with Padding logic is causing performance regression as the extra computation overtakes the existing compute cost.