docs: clarify Ledger product journey

[tcconnally]

Scope

• Name the current Perseus Context Engine explicitly in the product-family table.
• Link Ledger to the Context Engine, Vault, API reference, and benchmark evidence.
• Remove a volatile fixed MCP-tool count.
• Replace named customer/model-looking sample values with clearly synthetic examples.

Evidence

• Full local test suite: 919 passed; git diff --check passed.
• No runtime or API behavior changed.

Boundaries

• Example values are intentionally non-customer placeholders.
• No compatibility history or release metadata was removed.