feat(sequence): Phase 1 - Sequence v2 foundation

- Add Node system with typed buffers (u8x4_norm, f32x4, f16x8, depth24)
- Add NodeRegistry with aliasing support for ping-pong optimization
- Add SequenceV2 base class with DAG execution
- Add EffectV2 base class with multi-input/multi-output
- Add comprehensive tests (5 test cases, all passing)
- Corrected FATAL_CHECK usage (checks ERROR conditions, not success)

Phase 1 complete: Core v2 architecture functional.
Next: Phase 2 compiler (seq_compiler_v2.py)

handoff(Claude): Phase 1 foundation complete, all tests passing (35/35)

1 files changed, 93 insertions, 0 deletions

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+# Sequence v2 System Documentation
+
+This document describes the high-level ideas for an improved Sequence + Effects system "version 2".
+
+## Goal
+
+* more flexibility and less boilerplate
+* fully configurable by text (.seq)
+* no compatibility with previous version of sequences
+* sequence can be stacked (with priority) and arranged in a timeline to create a demo
+
+## Structure
+
+### Sequence
+
+A 'Sequence' unit consist of:
+  * A predeclared set of named assets and Effects that are used during the sequence
+  * a start-time, end-time
+  * a globally-visible set of internal time-dependent params (UniformsSequenceParams) derived from GlobalParams
+  * a globally-visible set of fixed params values (UniformsSequenceStaticParams) used to configure effects
+  * an input in RGBAu8 format ("Source"). Black by default (RGB=0,0,0,255). Buffer format: u8x4_norm (see below)
+  * an output in RGBAu8 format ("Sink"). This output goes to the next sequence in stack or to physical display
+  * the sink of a Sequence is the source of the next Sequence in the stack, or goes to screen if it's the last sequence in stack
+  * three programatic sections: 'Preprocess', 'EffectFlow' (time-ordered set of effects), 'Postprocess'
+  * a set of internal buffers (Nodes = compute buffers or textures, used as input or render targets by effects stack)
+  * this Nodes are visible to the final post-process effect
+  * Node persists during the whole sequence
+  * at compile time, one can detect which Node can actually be ping-pong buffers
+  * each Sequence's preprocess and postprocess is unique and attached to the sequence
+
+### Effects
+
+The effect unit consists in:
+  * An 'Effect' within the Sequence uses the UniformsSequenceParams and GlobalParams to update its internal state
+  * Receives updated InputNode from the previous effects (InputNode = buffer, textures, g-buffer, ...)
+  * processes them with a shader or c++ code (3d engine, etc.)
+  * fills its OutputNode, passed to the next effect or the final postprocessing step
+  * uses the sequence's assets and params only
+  * if needed an effect predefines an alias of one of its internal buffer as its declared 'postprocess_output' OutputNode
+  * by default, postprocess_output = sequence RGBA Source
+  * Effect's are not attached to a particular Sequence, and can be used in any Sequence
+
+### Preprocess:
+  * the preprocessing step sets up the local UniformsSequenceParams (prepare timers, objects' transforms, camera position, fills Uniform buffers, etc.) 
+  * it's a single function attached to the Sequence unit (virtual method?)
+
+### Postprocess:
+  * the post-process unique (virtual) function is responsible for the final assembly into OuputNode
+  * it can take any internal Node as input (most likely the last effects' output) and produces its final Sink content
+
+## How it works
+
+### initialization
+
+at initialization time, the sequence knows from compile-time:
+  * the list of assets it needs to be ready at start time
+  * the list of effects needed, their input nodes, their output node
+  * the list of internal textures, buffer, render target, depth-buffer it will need for its stack effect to work
+  * which intern Node buffers can actually be ping-pong buffers, to optimize resource
+  * generally speaking, the lifetime of a buffer during a frame is known at compile time and can be optimized
+
+### Flow
+  * preprocess: prepare internal state at time t: UniformsSequenceParams
+  * for each effects:
+     ** bind the InputNode nodes (previous effect buffers, compute buffers, etc.)
+     ** runs the compute, vertex and fragment passes
+     ** fills its output Nodes
+  * process: turns its pre-declared input Nodes into its unique Sink node, ready for next sequence, or display
+
+## Requirement
+
+  * unified code to flow the engine
+  * textual description of the sequence/effects arrangement (similar to the .seq)
+  * update the HTML editor tool
+  * unified description of buffer, texture and compute buffers at compilation time
+    ** example, of a sequence's internal Node declaration:
+     "NODE Name1 u8x16_norm"
+     "NODE Name2 f32x4"
+     "NODE Name3 f16x8"
+     "Node Name4 u8x4_norm" <- Source/Sink of a sequence, four bytes
+  * each effect
+    ** declares its requirement as input Node.
+    ** declares its output Node
+    ** can access the C++ version and WebGPU versions of GlobalParams, UniformsSequenceParams and UniformsSequenceStaticParams
+    ** are not attached to a particular sequence, but can be used in many
+  * validation and optimization at compile time (generating c++ code) by seq_compiler
+  * compilation can generate boilerplate preprocess() / postprocess() functions  
+  * no backward compatibility needed. Sequence v1 can go.
+  * the HTML Editor overall code, look and functionalities need to be preserved though. Just adapted to v2.
+    ** basically the sequence box will need to show input Node and output Node and have an editable property panel for these
+    ** same for effects: ability to edit their input Node and output Node names
+  * a lot of v1 effects can be deleted (solarized, chroma aberration, etc.): they will be implemented ad-hoc in postprocess() with a single macro call within the final shader
+  * need a minimal migration plan for code.