fix(#232): export memory cues to PCOB2 in DAT and EXT ANLZ files

[Radexito]

Summary

• Memory cues now export correctly to both CDJ hardware and Rekordbox PC. Previously, tracks with memory cues (set via hot_cue_index = -1) produced an empty PCOB2 section in all ANLZ files — CDJs and Rekordbox silently ignored them.
• Root cause: buildPcobSections and buildExtPcobSections in anlzWriter.js always returned EMPTY_PCOB_2 as a placeholder while memory cue format was under investigation (fix: memory cues not showing in Rekordbox PC (PCOB2 reverse engineering) #208). The underlying buildPcobSlot already handled slotType=0 / hot_cue_num=0 correctly — memory cues just needed to be wired through.
• Critical fix: native capture 42 revealed that PCOB2 with memory cue entries requires mem_count=0x00000000, not 0xFFFFFFFF (the hot-cue sentinel). Writing the wrong value caused Rekordbox to reject the entire DAT file, hiding waveform, beatgrid, and all analysis data.
• DAT PCOB2: memory cues now written for CDJ hardware playback.
• EXT PCOB2: memory cues now written for Rekordbox PC display.
• EXT PCO2 slot 2: was already handled by buildPco2Sections — no change needed.
• Tests updated: replaced the "PCOB2 always empty" assertions with correct coverage of memory cue encoding and mem_count values.

Files changed

• src/audio/anlzWriter.js — wire memory cues into PCOB2 for both DAT and EXT; fix mem_count sentinel
• src/__tests__/anlzWriter.test.js — update PCOB2 assertions to verify memory cue output and mem_count values

Test plan

• All 385 main-process unit tests pass
• Import a track and set at least one memory cue in DjManager
• Export to Rekordbox USB
• Open the USB in Rekordbox PC — memory cue appears in the cue list
• Verify hot cues (A–H) are unaffected and still appear correctly alongside memory cues
• Load the USB on a CDJ — memory cue marker should appear on the waveform/jog wheel

Closes #232

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