feat: auto-restart crashed LS with exponential backoff (#163 by @you922)

* feat: auto-restart crashed language server with exponential backoff

When a language server instance crashes, the proxy now automatically
respawns it after a brief backoff instead of leaving downstream
requests hanging with ECONNRESET / ERR_HTTP2_CONNECT errors.

Implementation:
- scheduleLsRestart() in langserver.js with configurable exponential
  backoff (base: 1s → 2s → 4s → 8s)
- Per-key retry counter to prevent infinite loops on permanent errors
- Env-configurable: LS_AUTO_RESTART, LS_AUTO_RESTART_MAX_RETRIES,
  LS_AUTO_RESTART_BASE_DELAY_MS
- getRestartStats() for monitoring restart health
- All timeouts use .unref() to not block process exit

Configuration:
  LS_AUTO_RESTART=1                   # default: on
  LS_AUTO_RESTART_MAX_RETRIES=3       # default: 3
  LS_AUTO_RESTART_BASE_DELAY_MS=1000  # default: 1000ms

Fixes: #157

* fix: prevent auto-restart on intentional LS shutdown (Oracle review)

Added _intentionalShutdown Set to track LS instances being shut
down deliberately via stopLanguageServer() or restartLsForProxy().
Without this, the exit handler unconditionally fires scheduleLsRestart
for every killed process, spawning unwanted LS instances.

Changes:
- _intentionalShutdown Set prevents restart for intentional kills
- restartLsForProxy marks key before process.kill
- stopLanguageServer marks all keys before kill loop
- Exit handler checks Set before calling scheduleLsRestart
- Cleanup after exit check (prevents set growth)

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Co-authored-by: Chen Jiangjiang <chenjiangjiang@kt51.cc>