Evolve token accounting

Goal

Evolve Mecatl token accounting from raw, opaque token totals into configurable cost-aware budgeting and understandable Mecatui usage reporting.

Scope

This parent tracks the coordinated work:

• Configure effective per-run and team token ceilings for embedded mecatui (#686).
• Add fixed-point, configurable weighted-cost accounting with correct cache-read/cache-write partitioning (#687).
• Display effective limits and budget state in Mecatui (#688).
• Add an interactive /tokens explanation and delegated-work breakdown with a non-double-counted grand total (#693).

Sequencing

1. Define durable raw usage and weighted-cost semantics, including cache-write treatment and configuration (#687).
2. Expose the relevant embedded-server configuration (#686).
3. Project effective budget and cost information through the server/UI boundary (#688).
4. Build the interactive usage panel and footer discoverability on that authoritative projection (#693).

The raw token facets remain valuable observability. Cost-weighted accounting must be a separately named value until any public compatibility/migration decision is made.

Start with the sequencing and the referenced issues, especially #687 for raw usage and weighted-cost semantics, then review #686, #688, and #693 in order. Done means the coordinated work defines durable accounting, exposes effective limits, projects authoritative budget data, and presents a non-double-counted usage breakdown.