feat(compiler): complete bitstream integration — all 9 changes
- AST: StreamDecl, StreamMode, Declaration::Stream, StreamFrom struct variant
- Lexer: Pixel, Delta, Signals keywords
- Parser: parse_stream_decl with mode block, fixed TokenKind::On match
- Signal graph: streamable flag, SignalManifest, Declaration::Stream detection
- Checker: StreamFrom { source, .. } pattern
- Codegen: DS._initStream(), DS._connectStream(), DS._streamDiff() hooks
- Runtime JS: full streaming layer with binary protocol encoding
- Layout: to_bytes/from_bytes on LayoutRect
82 tests pass (5 new: 3 parser stream + 2 analyzer streamable)
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