docs: backlog shipped hardening and lay out Phase 4-6 in ROADMAP

[TMHSDigital]

Summary

The ROADMAP claimed v0.8.0 (Phase 3 complete) and listed only cross-platform as "next", while the code has shipped through v0.8.8 with twelve hardening rounds. This refreshes it to match reality and sketches the next feature phases.

Changes

• Phases 1-3 marked shipped.
• New "Hardening" section backlogs the v0.8.1+ correctness rounds by issue (Bug: a second server instance can kill another instance's active recording #16-Enhancement: temp filter/list files can leak into the edits dir on a hard crash #24, Bug: region recording with odd width/height fails with a cryptic encoder error #34, Bug: sample_frames (timestamps) returns paths for frames past the end of the video #35, Bug: monitor:N capture is cropped/wrong on DPI-scaled displays (logical vs physical pixels) #39-Bug: negative / off-desktop region offsets are accepted but fail in ffmpeg with a cryptic error #41), and lists the open items: Bug: odd output dimensions cause a cryptic encoder error in re-encode tools (crop/scale/assemble_highlights/title_card) #47 (odd output dims), Bug: concat fails on relative input paths (list file lives in the OS temp dir) #48 (concat relative paths), Bug: extract_audio 'copy' writes a broken file for non-mp4 audio codecs #36 (extract_audio copy container), Enhancement: tools silently overwrite an existing output path #37 (overwrite protection).
• Phase 4 -- Cross-platform capture (next): expanded with concrete steps (avfoundation, x11grab/PipeWire, a per-platform enumerator behind the Provider seam, capability reporting, CI smoke tests).
• Phase 5 -- Captions and annotation (proposed): burn-in subtitles, optional transcribe-to-SRT, zoom-to-cursor / cursor emphasis, callouts, keystroke overlay.
• Phase 6 -- Audio polish and delivery (proposed): loudnorm, denoise, thumbnail/contact-sheet, optimized GIF/WebP, chapter markers, a pipeline tool.
• Cross-cutting backlog: overwrite protection, absolute input-path resolution, CONTRIBUTING engines note.

New issues filed alongside this

• Bug: odd output dimensions cause a cryptic encoder error in re-encode tools (crop/scale/assemble_highlights/title_card) #47 -- odd output dimensions cause a cryptic encoder error in crop/scale/assemble_highlights/title_card (sibling of Bug: region recording with odd width/height fails with a cryptic encoder error #34)
• Bug: concat fails on relative input paths (list file lives in the OS temp dir) #48 -- concat fails on relative input paths (list file lives in the OS temp dir)

Note on versioning

No version bump: ROADMAP.md is not part of the published package files (only dist, assets, README, LICENSE), so bumping would republish an identical package. Happy to add a docs: patch bump if you'd rather keep every PR versioned.

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