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/allocation-indicators/spec.md

@@ -0,0 +1,74 @@
+# Allocation Indicators Specification
+
+## Overview
+
+This capability defines low-noise variance indicators for planning and execution surfaces.
+Indicators are decision aids, not decorative status coloring.
+
+## ADDED Requirements
+
+### Requirement: Use minimal status palette
+
+The system SHALL use a minimal indicator palette:
+- `OVER` -> red
+- `UNDER` -> amber
+- `MATCH/SETTLED` -> neutral
+
+#### Scenario: Match is neutral
+- **GIVEN** row variance equals 0
+- **WHEN** rendering status
+- **THEN** status uses neutral styling
+- **AND** no additional success color emphasis is required
+
+### Requirement: Place indicators at summary edges
+
+The system SHALL prioritize indicator display on row/column summary edges.
+
+#### Scenario: Row-level over-allocation indicator
+- **GIVEN** project row total exceeds selected month plan
+- **WHEN** allocation grid renders
+- **THEN** project row summary status shows `OVER` in red
+
+#### Scenario: Column-level over-capacity indicator
+- **GIVEN** member column total exceeds member month capacity
+- **WHEN** allocation grid renders
+- **THEN** member column summary status shows `OVER` in red
+
+#### Scenario: Under-allocation indicator
+- **GIVEN** row or column total is below comparison target
+- **WHEN** grid renders
+- **THEN** summary status shows `UNDER` in amber
+
+### Requirement: Keep indicators explainable
+
+The system SHALL provide text status labels with numeric deltas for accessibility and clarity.
+
+#### Scenario: Color is not sole signal
+- **WHEN** status is rendered
+- **THEN** UI includes text label (`OVER`/`UNDER`) and delta value
+
+### Requirement: Distinguish project and resource variance semantics
+
+Project variance and resource variance SHALL remain separate.
+
+#### Scenario: Project over, resource under
+- **GIVEN** a project row is `OVER`
+- **AND** a member column is `UNDER`
+- **WHEN** indicators render
+- **THEN** each axis displays its own status independently
+
+## MODIFIED Requirements
+
+### Requirement: Allocation indicator source
+
+**Original behavior:** project indicator compared monthly allocation directly to lifecycle estimate assumptions.
+
+**Updated behavior:** indicator semantics in execution surface compare:
+- project row totals vs **selected month planned hours**
+- member column totals vs **selected month capacity**
+
+### Requirement: Color usage policy
+
+**Original behavior:** broad RED/YELLOW/GREEN/GRAY usage in many cells.
+
+**Updated behavior:** minimal red/amber/neutral policy with status emphasis on summary edges.