docs: Archive historical documentation (26 files → doc/archive/)

Moved completed/historical docs to doc/archive/ for cleaner context:

Archived (26 files):
- Analysis docs: variable tempo, audio architecture, build optimization
- Handoff docs: 6 agent handoff documents
- Debug reports: shadows, peak meter, timing fixes
- Task summaries and planning docs

Kept (16 files):
- Essential: AI_RULES, HOWTO, CONTRIBUTING, CONTEXT_MAINTENANCE
- Active subsystems: 3D, ASSET_SYSTEM, TRACKER, SEQUENCE
- Current work: MASKING_SYSTEM, SPECTRAL_BRUSH_EDITOR

Updated COMPLETED.md with archive index for easy reference.

1 files changed, 44 insertions, 0 deletions

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+# spectrogram editing tool in a browser (Task #5)
+
+The sub-project is about have an editing tool in a browser
+that will allow generating, editing, compacting .spec files
+
+## the idea
+
+We want a web page (html+javascript) capable of:
+   * loading a .wav file and showing the spectrogram (IDCT_SIZE=512)
+   * compacting this spectrogram if needed
+   * generating the .spec file
+   * allowing some editing on the spectrogram
+   * and most importantly: allowing the compression of this spectrogram using elementary bricks like bezier-curves (with a large stroke width /
+   brushes), rectangle, noise addition, etc.
+
+These tools must be available in the web editor (in javascript) as well as
+in c++ equivalent too.
+
+The idea is the compress the .wav spectrogram using elementary bricks and
+simple representation (as well as being dynamic) during the demo: we
+generate the audio on the fly by 'drawing' the spectrograms and passing them
+to the synth
+
+## the work
+
+Analyze the requirement for the HTML tool. It must remain simple and easy to
+use. Don't over-engineer it. - **Task #41: Spectrogram Editor HTML Tool**.
+Still, the tool must have a decent load/save offer, to:
+  * load the .wav
+  * save the .spec
+  * save/export the descriptive compressed version in vectorial form (like the SVG format, in fact)
+  * be able to load this vectorial form
+
+Then we need a reader for the vectorial form in c++ to use in the demo. - **Task #42: Vectorial Spectrogram Reader (C++)**.
+We also need to elementary tools / bricks to: draw a line / bezier curve /
+rectangle, ellipse, etc. in frequency domain, to generate the spectrogram on
+the flight. - **Task #43: Spectrogram Drawing Bricks**.
+We also need controllable random noise generation to add some 'texture' to
+the spectrogram.
+
+## the files
+the HTML/JS tool should be under tools/editor
+the c++ code should be added to the library under src/audio/
+