chore(mcp): small-surface residuals triage — USAspending, FRED series param, aircraft registry lookup, NGA nav warnings, global air snapshot

## Summary

Triage checklist of small agent-surface gaps from a full audit of our MCP/API coverage. Each item is individually too small for its own issue but none should be silently dropped: each gets promoted to a scoped issue, folded into an adjacent one, or closed here with an explicit wontfix reason.

## Items

- [ ] **US federal spending analytics** (USAspending.gov: top agencies by budget, per-agency drilldown). Adjacent to `get_procurement_opportunities` (which covers tender *opportunities*, not budget flows). Free API.
- [ ] **On-demand FRED series parameter** — we expose curated FRED-derived series (Fed Funds, curve inputs); a bounded `series_id` allowlist parameter would let agents pull common macro series (UNRATE, CPIAUCSL, T10YIE) without new seeders.
- [ ] **Aircraft registry lookup** — resolve ICAO24 hex → registration/type/operator (hexdb.io, free). `get_airspace` returns live positions; agents currently can't enrich an unknown hex. Single + small-batch form.
- [ ] **Raw NGA maritime navigational warnings** — `get_maritime_activity` serves derived disruption data; the underlying active NAVAREA warnings (free NGA feed) are not queryable. Useful primary-source citations for maritime analysis.
- [ ] **Global airborne-traffic snapshot** — total aircraft aloft + regional breakdown (we expose per-country `get_airspace`; a world-level rollup is a cheap aggregate of data we already touch).

## Rule

When closing an item, record the disposition inline (promoted → link issue; folded → link issue; wontfix → one-line reason). The checklist is done when no unchecked items remain.

Start by reviewing the five checklist items against the existing get_procurement_opportunities, get_airspace, and get_maritime_activity surfaces and their named upstream APIs. Done means each item is promoted with a linked issue, folded into an adjacent issue, or closed with an inline wontfix reason, leaving no unchecked boxes.

Written by the indexing model from the issue text.