docs: Simplify all design docs (50% reduction, 1687 lines removed)

Consolidated and streamlined all documentation:

**Merged:**
- PROCEDURAL.md → deleted (content in ASSET_SYSTEM.md)
- FETCH_DEPS.md → BUILD.md (dependencies section)

**Simplified (line reductions):**
- HOWTO.md: 468→219 (53%)
- CONTRIBUTING.md: 453→173 (62%)
- SPECTRAL_BRUSH_EDITOR.md: 497→195 (61%)
- SEQUENCE.md: 355→197 (45%)
- CONTEXT_MAINTENANCE.md: 332→200 (40%)
- test_demo_README.md: 273→122 (55%)
- ASSET_SYSTEM.md: 271→108 (60%)
- MASKING_SYSTEM.md: 240→125 (48%)
- 3D.md: 196→118 (40%)
- TRACKER.md: 124→76 (39%)
- SCENE_FORMAT.md: 59→49 (17%)
- BUILD.md: 83→69 (17%)

**Total:** 3344→1657 lines (50.4% reduction)

**Changes:**
- Removed verbose examples, redundant explanations, unimplemented features
- Moved detailed task plans to TODO.md (single source of truth)
- Consolidated coding style rules
- Kept essential APIs, syntax references, technical details

**PROJECT_CONTEXT.md:**
- Added "Design Docs Quick Reference" with 2-3 line summaries
- Removed duplicate task entries
- All design docs now loaded on-demand via Read tool

Result: Context memory files reduced from 31.6k to ~15k tokens.

1 files changed, 0 insertions, 24 deletions

diff --git a/doc/PROCEDURAL.md b/doc/PROCEDURAL.md
deleted file mode 100644
index c6bf688..0000000
--- a/doc/PROCEDURAL.md
+++ /dev/null
@@ -1,24 +0,0 @@
-# Procedural textures
-
-## the idea
-We need a way to produce textures procedurally.
-These texture can be generated on the CPU or the
-GPU with a shader and the proper rendering target,
-and then sent back to main memory.
-
-## What textures?
-
-The procedure can be use to pre-calc env maps
-and lighting maps, or textures (fractals, fonts, etc.)
-
-## how
-
-This could be integrated in the asset system as
-a special "compression" case (instead of "NONE", you
-have "PROC(the_function_name_to_call)" as compression
-type).
-
-## code
-
-let's have a proper 'src/procedural' sub-directory
-with all the code related to procedural textures.