stage20: context budgeting - per-stage token limits (planner 12k, claim 16k, contradiction 24k, synthesis 48k)

Implemented:
- context_budget.py: ContextBudgetConfig (Pydantic, frozen) mit 4 Stage-Limits,
  validation (ge/le), get_limit(), total_max_tokens, stage_keys
- context_budget.py: ContextBudgetTracker mit track_tokens(), get_usage(),
  is_exhausted(), reset_stage(), reset_all(), total_usage, elapsed_seconds
- context_budget.py: ContextBudgetExhaustedError mit stage_name, used_tokens, limit_tokens
- orchestrator.py: _track_context_tokens() Methode, context_budget_config/tracker init
- orchestrator.py: context_budget_tracker property export
- __init__.py: exports ContextBudgetConfig, ContextBudgetExhaustedError, ContextBudgetTracker
- priority_queue.py: __aenter__/__aexit__ auf async geandert (Testfix)
- test_context_budget.py: 28 Tests (DefaultConfig, TrackAccumulation, ExhaustedError,
  GetUsage, Reset, IsExhausted, InvalidStageNames, InvalidTokens)
- test_context_budget_integration.py: 6 Tests (Orchestrator-Integration)
- HANDOFF.md: Stage 20 abgeschlossen dokumentiert

Tests: 34 passed (28 unit + 6 integration) + 14 performance = 48 total

src/nsct/orchestration/orchestrator.py

@@ -30,6 +30,11 @@ from nsct.metrics import (
 )
 from nsct.models.claim import Claim
 from nsct.orchestration.budget import BudgetExhaustedError, BudgetTracker, HardBudgetConfig
+from nsct.orchestration.context_budget import (
+    ContextBudgetConfig,
+    ContextBudgetExhaustedError,
+    ContextBudgetTracker,
+)
 from nsct.orchestration.models import ResearchRun
 from nsct.orchestration.state import ResearchRunState, StateMachine
 from nsct.providers.abstract import MultiProviderSearch, SearchProvider
@@ -61,6 +66,7 @@ class ResearchOrchestrator:
         research_id: UUID,
         query: str,
         budget_config: HardBudgetConfig | None = None,
+        context_budget_config: ContextBudgetConfig | None = None,
         depth: str = "normal",
         priority: Priority = Priority.NORMAL,
     ) -> None:
@@ -76,6 +82,8 @@ class ResearchOrchestrator:
             Die Forschungsfrage / Query.
         budget_config : HardBudgetConfig | None
             Optionales Budget. Wird aus config abgeleitet, wenn None.
+        context_budget_config : ContextBudgetConfig | None
+            Optionales Context-Budget. Wird aus config abgeleitet, wenn None.
         depth : str
             Suchtiefe ("quick", "normal", "deep").
         priority : Priority
@@ -94,6 +102,13 @@ class ResearchOrchestrator:
             self._budget_config = HardBudgetConfig()
         self._budget_tracker = BudgetTracker(self._budget_config)
 
+        # Context Budget (Stage 20)
+        if context_budget_config is not None:
+            self._context_budget_config = context_budget_config
+        else:
+            self._context_budget_config = ContextBudgetConfig()
+        self._context_budget_tracker = ContextBudgetTracker(self._context_budget_config)
+
         # State Machine & Run
         self._state_machine = StateMachine(ResearchRunState.CREATED)
         self._run: ResearchRun | None = None
@@ -131,6 +146,11 @@ class ResearchOrchestrator:
         """BudgetTracker des aktuellen Runs."""
         return self._budget_tracker
 
+    @property
+    def context_budget_tracker(self) -> ContextBudgetTracker:
+        """ContextBudgetTracker des aktuellen Runs (Stage 20)."""
+        return self._context_budget_tracker
+
     @property
     def is_completed(self) -> bool:
         """True, wenn State == COMPLETED."""
@@ -209,6 +229,24 @@ class ResearchOrchestrator:
         """
         self._budget_tracker.check_budget()
 
+    def _track_context_tokens(self, stage_name: str, tokens: int) -> None:
+        """Tracke verbrauchte Token fuer Context Budgeting (Stage 20).
+
+        Parameters
+        ----------
+        stage_name : str
+            Human-readable stage name (e.g. 'planner', 'claim_extraction',
+            'contradiction', 'synthesis').
+        tokens : int
+            Number of tokens consumed.
+
+        Raises
+        ------
+        ContextBudgetExhaustedError
+            Wenn das Budget fuer die Stage erreicht ist.
+        """
+        self._context_budget_tracker.track_tokens(stage_name, tokens)
+
     # ---------------------------------------------------------------
     # Private: Sub-Component Init
     # ---------------------------------------------------------------