Adds "direct-to-decode" to validDisaggregationDeciders map and adds
factory panic case directing callers to use NewDirectToDecodeDecider.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Adds PDDirectDecodeThreshold field to DeploymentConfig, switch-based
decider construction in NewClusterSimulator, and updates interference
warning to only fire for fully-disaggregated deployments (decider=always).

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

…ts (INV-P2-4a)

When pools are configured and a request is not disaggregated, route it
to the decode pool only via a new poolFilter field on RoutingDecisionEvent.
Decode instances handle both phases with interference cost from PR3.

BC-P2-14: non-disaggregated + pools → decode pool only
BC-P2-15: non-disaggregated → no ParentRequest records

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Registers the --pd-direct-decode-threshold flag (default 256) and wires
it to DeploymentConfig. Includes validation (>= 0) and stale-flag
warning when used with a different decider.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Verifies short prompts route directly to decode pool while long prompts
go through full PD pipeline (BC-P2-14, BC-P2-15, BC-P2-16, INV-1).

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

…decode

- INV-P2-4a: non-disaggregated + pools → decode pool (tested with never decider)
- INV-P2-4b/BC-P2-17: interference applied to mixed-phase decode batches
- INV-6: determinism for mixed short/long workloads
- BC-P2-13: always-disaggregate behavior unchanged by pool filter

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

- Add --pd-direct-decode-threshold to CLI flags listing
- Add DirectToDecodeDecider to disaggregation.go file description
- Add PDDirectDecodeThreshold to DeploymentConfig description
- Add INV-P2-4 (decode-targeted routing) to invariants section
- Update disaggregated data flow with direct-to-decode decider option
- Update local path to show decode pool routing (INV-P2-4)

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

…delines update

- invariants.md: add INV-P2-3 (interference monotonicity) and INV-P2-4
  (decode-targeted routing) with verification strategies
- configuration.md: add direct-to-decode decider to table, semantics
  section, and --pd-direct-decode-threshold flag
- design-guidelines.md: add DisaggregationDecider to Section 4.2 module
  map with all four variants

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

- TestDirectToDecodeDecider_ClusterConstruction: replace nil check with
  behavioral assertion (decode pool routing) per BDD/TDD principles
- TestDirectToDecodeDecider_MixedWorkload: tie INV-1 check to request
  slice lengths instead of hardcoded constant; clarify sub-request
  vs parent-request counting in comment
- buildPoolFilteredSnapshots: only construct snapshots for target pool
  members; eliminates O(N/2) wasted Snapshot() calls per routing event
- DisaggregationDecisionEvent.Execute: document no-pools fallback path
- DirectToDecodeDecider.Decide: add empty-input note to method docstring

Co-Authored-By: Claude Sonnet 4.6 (1M context) <[email protected]>

Addresses findings from comprehensive PR review:

Safety (C1, C3, C6, C8):
- C1: Add PDTransferBaseLatencyMs upper-bound guard (<=3.6e9 ms) in
  NewClusterSimulator to prevent int64 overflow in transfer duration
  calculation (silent 1 µs clamp bug)
- C3: Add PoolOverrides.Validate(name) method enforcing *TP>0,
  *MaxModelLen>0, *TotalKVBlocks>0; call from NewClusterSimulator
  before instance construction (R3)
- C6: Add BlockSizeTokens>0 check in NewClusterSimulator PD block so
  error appears at construction time, not as a panic inside Run() (R6)
- C8: Add clarifying comments above unreachable panic sites in
  newInstanceSimulatorCore explaining why they're safe and what to do
  if the function is ever called outside NewClusterSimulator

Documentation (C4, C5):
- C4: Update NewDisaggregationDecider docstring to include
  "direct-to-decode" in panic description
- C5: Update PDDecider field comment to include "direct-to-decode"
  in the valid-values enum

Test coverage (GAP-1, GAP-2, GAP-3):
- GAP-1: Add INV-P2-4 pool-membership assertion to
  TestPrefixThreshold_HighThresholdNoDisaggregation
- GAP-2: Add TestCollectPDMetrics_DroppedAtDecodeKV table-driven unit
  test for the TransferCompleteTime>0 && DecodeInstanceID=="" condition
- GAP-3: Add INV-P2-4 pool-membership assertion to
  TestDisaggregationDecisionEvent_SchedulesRouting

Co-Authored-By: Claude Sonnet 4.6 (1M context) <[email protected]>

…erage

Performance: detectPrefillCompletions/detectDecodeCompletions O(N×M) → O(K)
via pendingPrefillByInstance/pendingDecodeByInstance per-instance indexing.
Pending maps cleared at simulation end to release memory for dropped/horizon-
truncated requests.

Validation: warn when prefill+decode < total instances (unassigned idle).

Tests: TestDisaggregation_MaxModelLen_DropsOversizedRequests — exact drop count,
CompletedRequests verification, and INV-1 conservation check.

Docs: length_capped_requests anomaly counter; README "Four latency modes";
quickstart PD examples; tutorial Step 8 PD break-even walkthrough;
extension-recipes tier-composition pattern; pd-disaggregation-demo.yaml.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <[email protected]>

…rage

Co-Authored-By: Claude Sonnet 4.6 (1M context) <[email protected]>

…pshot, contention accuracy, and docs

Co-Authored-By: Claude Sonnet 4.6 (1M context) <[email protected]>

…, and warning accuracy

- sim/disaggregation.go: add nil guard in DirectToDecodeDecider.Decide (panics with
  named message instead of NPE deep in event loop)
- sim/cluster/cluster_event.go: fix spurious "unhandled pool role" warning — known
  roles without custom policies fall through silently; warning reserved for
  unrecognized roles only
- sim/cluster/cluster_event.go: replace dead-code-path comment "When pools are NOT
  configured" with accurate "Defensive: always true here" note
- cmd/root.go: fix misleading zero-threshold warning — qualifies "for non-empty
  inputs" and notes the empty-input exception
- cmd/root.go: fix topology guard (&&→||) to catch the one-pool-zero case with the
  friendly "requires both" message

Co-Authored-By: Claude Sonnet 4.6 (1M context) <[email protected]>