feat(mistral): expand to 13 namespaced metrics from existing payload

[anthonybaldwin]

Summary

Replaces the single session-percent metric on the Mistral provider with the v1 namespaced inventory from plans/mistral-tier-coverage.md. 13 metric IDs total. No new HTTP calls — the existing GET admin.mistral.ai/api/billing/v2/usage already returns every field; the parser was decoding 10+ values it then threw away.

• Costs (8): monthly-cost, monthly-cost-completion, monthly-cost-ocr, monthly-cost-audio, monthly-cost-connectors, monthly-cost-libraries, monthly-cost-fine-tuning, monthly-cost-vibe
• Tokens (3): monthly-input-tokens, monthly-output-tokens, monthly-cached-tokens (input metric carries a XX% cached caption when both >0)
• Account (2): model-count, period-end

Migration

Pinned buttons bound to the legacy session-percent auto-rebind to monthly-cost via the existing metricIDAliases map in cmd/plugin/main.go. handleWillAppear now persists the migrated ID back to KeySettings (read-side aliasing already worked) and logs a one-line summary on the first migration per plugin launch:

metricID migration: mistral/session-percent → monthly-cost (rebinding stale buttons silently)

PI changes

METRICS[\"mistral\"] in stat.html is grouped via a new [\"__group__:<label>\"] row marker. populateMetrics renders these as <optgroup> separators ("Costs" / "Tokens" / "Account"). Other providers continue to render as flat lists; behavior is opt-in per provider.

Currency

Payload symbol preserved verbatim (typically €). No synthetic USD conversion. NumericGoodWhen=\"low\" on every cost metric; left unset on model-count per the plan rationale (more models used isn't inherently better).

Tests

• Fixture at internal/providers/mistral/testdata/billing-v2-usage.json is ported from tmp/CodexBar/Tests/CodexBarTests/MistralUsageParserTests.swift with extra categories (OCR / connectors / libraries / fine-tuning / audio / vibe) added so the new per-category emission paths get exercised.
• Parser tests: token totals, per-category spend math, period-end parsing, empty response.
• Snapshot tests: all 13 metric IDs present, legacy session-percent gone, currency formatting, cached-ratio caption.
• cmd/plugin/main_test.go gains 3 cases locking in the mistral alias.

Out of scope

v2 (tier-progress, spend-limit-percent, le-chat-plan, le-chat-seats) — those need DevTools spelunking against undocumented /api/... paths. Documented as a follow-up in the plan.

Test plan

• go vet ./... clean
• golangci-lint run ./... clean (0 issues)
• go build ./... clean
• go test ./... passes (mistral tests new; cmd/plugin extended)
• stat.html JS parses (Function constructor sanity check)
• Manual: load PI for a Mistral button, confirm 13 metrics in 3 optgroups
• Manual: button bound to legacy session-percent auto-rebinds to monthly-cost after restart and the migration log line appears

🤖 Generated with Claude Code