feat(plugin-sdk): add frame pool allocation for native plugins

[staging-devin-ai-integration]

Summary

Zero-copy buffer semantics for native plugin video/audio frames via host-side frame pool allocation. Eliminates per-frame memcpy on the hot path by letting plugins write directly into pool-allocated buffers and pass them back by handle.

DHAT profiling results (300-frame Slint watermark pipeline):

• Total allocations: 1,042,800,741 → 252,676,535 bytes (−75.8%)
• Target to_vec copy site (9,504,000 bytes / 300 blocks): eliminated entirely

Changes

SDK (sdks/plugin-sdk/native/src/)

• New PooledVideoBuffer / PooledAudioBuffer linear-type wrappers with Drop-based pool return
• OutputSender::alloc_video(), alloc_audio(), send_video(), send_audio() for zero-copy path
• CNodeCallbacks struct consolidating output + telemetry + alloc callbacks (replaces positional params)
• CNodeCallbacks.struct_size field for forward-compatible extensibility
• result_from_c() helper extracting duplicated CResult→Result conversion
• CAllocVideoResult / CAllocAudioResult allocation result types
• Buffer types re-exported in prelude
• API version bump: 5 → 6

Host (crates/plugin-native/src/wrapper.rs)

• CallbackContext extended with VideoFramePool / AudioFramePool
• alloc_video_shim / alloc_audio_shim / free_*_buffer C callbacks
• free_packet_buffer_handle() safety net: frees pooled handles on callback error paths
• build_node_callbacks() constructs the consolidated struct with struct_size

Conversions (sdks/plugin-sdk/native/src/conversions.rs)

• packet_from_c: zero-copy branch reclaims PooledVideoData/PooledSamples via Box::from_raw
• packet_from_c: null-data/null-samples error paths now free buffer_handle before returning (fixes buffer leak)
• packet_to_c: extended CPacketOwned::Video/Audio with metadata storage (prevents dangling pointer)
• Regression tests for buffer-handle cleanup on null-pointer error paths

Slint plugin

• Uses pool allocation with automatic fallback to legacy copy path

Review feedback addressed

From reviewer:

• Critical fix(tools): install script #1: Fixed buffer leak in packet_from_c when data/samples is null but buffer_handle is non-null
• Critical docs: improve UI documentation #2: Fixed buffer leak in output_callback_shim when pin name parsing fails before packet_from_c runs
• Suggestion fix(plugins): resolve flaky native plugin reload test #3: Removed redundant data_len/sample_count params from send_video/send_audio — now uses buf.len()/buf.sample_count()
• Suggestion docs: remove redundant note #4: Removed const fn from as_mut_slice/consume — these dereference heap pointers and will never be called in const context. Added #[allow(clippy::missing_const_for_fn)] with rationale.
• Suggestion Built-in authentication #5: Added struct_size to CNodeCallbacks for v7-on-v6 forward compat
• Suggestion fix(docs): http API inaccuracy #6: Extracted result_from_c() helper for CResult→Result conversion (4 call sites)
• Nit: Renamed CAllocResult → CAllocVideoResult
• Nit: Added PooledVideoBuffer/PooledAudioBuffer to prelude
• !Send: Acknowledged — both buffer types are !Send due to raw pointers, consistent with OutputSender. If cross-thread buffer passing is needed later, explicit unsafe impl Send would be required.

From Devin Review:

• Fixed resource leak in alloc_video/alloc_audio when free_fn is None but allocation succeeded — now checks free_fn.is_none() alongside data.is_null()

Additional:

• Downgraded info! → trace! for debug logging in __plugin_flush macro (4 lines that would spam production logs)

Review & Testing Checklist for Human

• Buffer ownership across FFI: Verify Box::into_raw → Box::from_raw round-trip in alloc_video_shim → packet_from_c is sound (the Vec heap pointer survives the Box indirection)
• Error-path cleanup: Confirm free_packet_buffer_handle in output_callback_shim correctly handles all error paths where buf.consume() has already suppressed Drop
• struct_size field: Verify CNodeCallbacks struct layout with the new leading struct_size: usize field doesn't break ABI alignment for any callback pointer fields
• Backward compat: Build a plugin against SDK v6 with alloc_video returning null (no pool) — should fall through to legacy .to_vec() copy path
• End-to-end: Run Slint watermark pipeline and verify video output is correct with pool allocation active

Notes

• All 10 non-Slint native plugins compile unchanged against the new API — the CNodeCallbacks consolidation is handled transparently by the macros.

Link to Devin session: https://staging.itsdev.in/sessions/ed6a1517580e43839384cd56691bf505
Requested by: @streamer45

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 6 additional findings.

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