docs(openspec): add reporting API contract documentation

Add comprehensive API documentation for the reporting endpoint:
- Request/response structure
- View type inference (did/is/will)
- Blank vs explicit zero semantics
- Status values and error responses

Related to enhanced-allocation change.

openspec/changes/enhanced-allocation/specs/resource-allocation/spec.md

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+# Resource Allocation - Delta Specification
+
+This delta documents required updates to existing resource allocation behavior for fidelity with explicit month planning.
+
+## MODIFIED Requirements
+
+### Requirement: Month execution comparison target
+
+**Original behavior:** month execution was compared against derived or lifecycle assumptions.
+
+**Updated behavior:** selected month project allocation is compared against explicit project-month planned hours.
+
+#### Scenario: Compare row total to month plan
+- **GIVEN** selected month plan for project is 1200h
+- **AND** project allocations total 1300h
+- **THEN** project row variance is +100h
+- **AND** row status is `OVER`
+
+#### Scenario: Blank month plan comparison
+- **GIVEN** selected month has no plan value set
+- **AND** project allocations total 50h
+- **THEN** comparison target is 0h
+- **AND** row status is `OVER`
+- **AND** allocation remains allowed
+
+### Requirement: Bulk allocation behavior
+
+**Original behavior:** all-or-nothing transaction semantics.
+
+**Updated behavior:** valid items SHALL be saved even if some items fail.
+
+#### Scenario: Partial bulk success
+- **WHEN** 10 allocation items are submitted and 2 fail validation
+- **THEN** 8 valid items are persisted
+- **AND** failed items return per-index validation errors
+- **AND** response includes summary created/failed counts
+
+### Requirement: Untracked execution semantics
+
+**Original behavior:** untracked support was ambiguous/incomplete.
+
+**Updated behavior:** `team_member_id = null` is valid and treated as untracked effort.
+
+#### Scenario: Untracked counted in project, excluded from capacity
+- **WHEN** untracked allocation exists for selected month
+- **THEN** project totals include it
+- **AND** member capacity/utilization computations exclude it