Consolidate Goldsky endpoint and schema reference and self-document

[timfong888]

Problem

There is no single file that defines both the Goldsky subgraph endpoints AND the schema entities used by the explorer. An AI agent or new developer must piece together information from multiple files:

• lib/config/network.ts — endpoint construction (project ID, subgraph names, versions)
• lib/graphql/client.ts — GraphQL client configuration
• METRIC-DEFINITIONS.md — metric formulas and sample queries (has a "Data Sources" section but it's incomplete)
• subgraph/README.md — generic deployment guide with no endpoints or schema

METRIC-DEFINITIONS.md already has the right structure but has gaps — notably line 508: {fill this in based on code}.

Proposed Changes

Update METRIC-DEFINITIONS.md to be the single canonical reference:

1. Add an Endpoints table at the top of the Data Sources section covering all networks:

Network	Subgraph	Version	Endpoint URL	Contract Address
Mainnet	filecoin-pay-mainnet	x.y.z	full URL	0x...
Mainnet	fwss-mainnet	x.y.z	full URL	0x...
Calibration	filecoin-pay-calibration	x.y.z	full URL	0x...
Calibration	fwss-calibration	x.y.z	full URL	0x...

2. Fill in the incomplete FWSS section (currently {fill this in based on code})
3. Add PDP Explorer endpoint — already used in code but not formally documented
4. Cross-reference lib/config/network.ts as the runtime source of truth, so the doc and code stay in sync
5. Clarify official vs dev-fork endpoints — currently the code uses -tim suffixed subgraph names which are dev forks

Why This Matters

• AI agents (Claude Code, Cursor, etc.) need a single file to understand the data layer
• New contributors shouldn't have to read 4+ files to find an endpoint
• Metric definitions without endpoint context are incomplete

Acceptance Criteria

• All Goldsky endpoints (mainnet + calibration) listed in one table
• FWSS data source section filled in
• PDP Explorer endpoint documented
• {fill this in based on code} placeholder removed
• Doc references lib/config/network.ts as code source of truth