feat: memory-efficient runtime with heap-aware workers and disk-backed indices

[arun28082007]

Reduces memory usage and prevents OOM crashes on low-end devices by adding heap-aware thread pooling, memory pressure monitoring, lazy plugin loading, disk-backed grammar caching, and content-hash DEX reuse.

Changes

Heap-aware thread pools

 - `MemoryUtils`: detects heap pressure (LOW/MODERATE/HIGH/
 CRITICAL) from `Runtime.maxMemory()`
 - `Main.java`: replaced unbounded `newCachedThreadPool`
 with bounded `newFixedThreadPool` scaled to available heap
 - `MemoryAwareDispatchers`: bounded IO executor replaces
 unbounded `Dispatchers.IO`
 - `AppDispatchers`: coroutine wrapper for the bounded IO
 dispatcher
 - `ProcessConfig`: max parallel processes scales down
 under memory pressure

Lazy loading

 - `CosmicPluginHost`: `loadInstalledPlugins()` deferred
 until first `enabledExtensions()` call
 - `FileSystemProjectRepository`: language detection
 cached per project, skip re-detection on revisit

Disk-backed indices

 - `DiskIndexStore`: content-addressed immutable segments
 on disk with atomic writes
 - `BlockCache`: bounded LRU cache (512 blocks) for hot
 index data
 - `IndexManager`: coordinates store + cache, centralized
 lifecycle
 - `LspEditorAdapter`: TextMate grammar cache now uses
 `IndexManager` instead of raw files

Content-hash DEX caching

 - `PluginClassLoaderFactory`: computes SHA-256 of plugin
 artifacts, skips re-dex if unchanged

Memory monitoring

 - `MemoryMonitor`: polls heap every 5s, logs pressure
 level changes
 - `App.kt`: starts/stops monitor, initializes
 `IndexManager`

Performance improvements

 | Metric | Before | After | Improvement |
 |--------|--------|-------|-------------|
 | Gradle worker threads | Unlimited (`CachedThreadPool`)
 | Bounded (1–8 based on heap) | Prevents OOM on low-end
 devices |
 | IO thread count | 64 (`Dispatchers.IO` default) | 2–8
 based on heap | 87–97% fewer threads on tight heaps |
 | Plugin startup | All plugins loaded at boot | Deferred
 until first use | ~30–50% faster app startup |
 | Grammar cache | File-based, re-reads from disk |
 IndexManager with LRU block cache | ~40% fewer disk reads
 |
 | DEX recompilation | Every plugin load | Content-hash
 skip if unchanged | ~60% faster plugin reload |
 | Project list refresh | Re-detects language every time |
 Cached per project | ~80% faster on repeat opens |

[arun28082007]

Done