Reactive: add refined stats tracking, remove V1, rename ReactiveV2 to Reactive

- Add adds_received/removes_received to track input churn
- Add adds_emitted/removes_emitted to track output changes
- Add process_count (runs) to track how many times each node processes
- Delete unused V1 API (Reactive.ml/mli)
- Rename ReactiveV2 to Reactive throughout codebase
- Split ReactiveTest.ml into themed test files
- Add Makefile and README for analysis/reactive
- Clean up README to remove V1/V2 references

Author: cristianoc

analysis/reactive/Makefile

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+.PHONY: build test clean
+
+# Build the reactive library
+build:
+	dune build src/
+
+# Run all tests
+test:
+	dune build test/ReactiveTest.exe
+	dune exec test/ReactiveTest.exe
+
+# Clean build artifacts
+clean:
+	dune clean
+

analysis/reactive/README.md

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+# Reactive Collections Library
+
+A library for incremental computation using reactive collections with delta-based updates.
+
+## Overview
+
+This library provides composable reactive collections that automatically propagate changes through a computation graph. When source data changes, only the affected parts of derived collections are recomputed.
+
+### Key Features
+
+- **Delta-based updates**: Changes propagate as `Set`, `Remove`, or `Batch` deltas
+- **Glitch-free semantics**: Topological scheduling ensures consistent updates
+- **Composable combinators**: `flatMap`, `join`, `union`, `fixpoint`
+- **Incremental fixpoint**: Efficient transitive closure with support for additions and removals
+
+## Usage
+
+```ocaml
+open Reactive
+
+(* Create a source collection *)
+let (files, emit) = source ~name:"files" ()
+
+(* Derive collections with combinators *)
+let decls = flatMap ~name:"decls" files
+  ~f:(fun _path data -> data.declarations)
+  ()
+
+let refs = flatMap ~name:"refs" files
+  ~f:(fun _path data -> data.references)
+  ~merge:PosSet.union
+  ()
+
+(* Join collections *)
+let resolved = join ~name:"resolved" refs decls
+  ~key_of:(fun pos _ref -> pos)
+  ~f:(fun pos ref decl_opt -> ...)
+  ()
+
+(* Compute transitive closure *)
+let reachable = fixpoint ~name:"reachable"
+  ~init:roots
+  ~edges:graph
+  ()
+
+(* Emit changes *)
+emit (Set ("file.res", file_data))
+emit (Batch [set "a.res" data_a; set "b.res" data_b])
+```
+
+## Combinators
+
+| Combinator | Description |
+|------------|-------------|
+| `source` | Create a mutable source collection |
+| `flatMap` | Transform and flatten entries, with optional merge |
+| `join` | Look up keys from left collection in right collection |
+| `union` | Combine two collections, with optional merge for conflicts |
+| `fixpoint` | Compute transitive closure incrementally |
+
+## Building & Testing
+
+```bash
+# Build the library
+make build
+
+# Run all tests
+make test
+
+# Clean build artifacts
+make clean
+```
+
+## Test Structure
+
+Tests are organized by theme:
+
+| File | Description |
+|------|-------------|
+| `FlatMapTest.ml` | FlatMap combinator tests |
+| `JoinTest.ml` | Join combinator tests |
+| `UnionTest.ml` | Union combinator tests |
+| `FixpointBasicTest.ml` | Basic fixpoint graph traversal |
+| `FixpointIncrementalTest.ml` | Incremental fixpoint updates |
+| `BatchTest.ml` | Batch processing tests |
+| `IntegrationTest.ml` | End-to-end file processing |
+| `GlitchFreeTest.ml` | Glitch-free scheduler tests |
+
+## Glitch-Free Semantics
+
+The scheduler ensures that derived collections never see inconsistent intermediate states:
+
+1. **Topological levels**: Each node has a level based on its dependencies
+2. **Accumulate phase**: All deltas at a level are collected before processing
+3. **Propagate phase**: Nodes process accumulated deltas in level order
+
+This prevents issues like:
+- Anti-joins seeing partial data (e.g., refs without matching decls)
+- Multi-level unions causing spurious additions/removals
+
+## Usage in Reanalyze
+
+This library powers the reactive dead code analysis in reanalyze:
+
+- `ReactiveFileCollection`: Manages CMT file processing
+- `ReactiveMerge`: Merges per-file data into global collections
+- `ReactiveLiveness`: Computes live declarations via fixpoint
+- `ReactiveSolver`: Generates dead code issues reactively
+