Add complete OpenRTB 2.6 data model crate

[ChristianPavilonis]

Summary

• Introduces a new trusted-server-openrtb crate containing a complete OpenRTB 2.6 data model (all request/response objects with every field from the spec) to replace the minimal hand-rolled structs in crates/common/src/openrtb.rs.
• Refactors crates/common to use the new crate's types via builder functions, keeping call sites clean while gaining full spec coverage (video, audio, native, DOOH, supply chain, EIDs, pod bidding, etc.).
• Populates previously-unmapped OpenRTB fields from data already available in the auction request/context, improving bid quality and compliance.
• Adds an openrtb.md reference document covering the full OpenRTB 2.6 schema for contributor use.

OpenRTB Field Enrichment

The Prebid integration now maps the following fields that were previously left empty despite data being available:

OpenRTB Field	Source	Impact
Imp.bidfloor / bidfloorcur	AdSlot.floor_price	Floor-price-aware bidders can now respect publisher floors
Imp.secure	Hardcoded 1	Ensures HTTPS-only creatives
Imp.tagid	slot.id	Gives bidders a stable slot identifier
Geo.lat / lon	GeoInfo.latitude / longitude	Improves geo-targeted demand
Geo.metro	GeoInfo.metro_code (DMA)	Critical for US-targeted campaigns
User.consent	UserInfo.consent (TCF string)	GDPR compliance — consent string now forwarded
Regs.gdpr	Set to 1 when consent present	GDPR compliance flag
Regs.us_privacy	Sec-GPC header → "1YYN"	Promoted from ext to first-class field
Device.dnt	DNT: 1 header	Do Not Track signal forwarded to bidders
Device.language	Accept-Language header	Language targeting for bidders
Site.ref	Referer header	Referrer URL in OpenRTB (not just HTTP header)
Site.publisher	Publisher domain	Canonical publisher object for bidders
BidRequest.tmax	config.timeout_ms	PBS can manage internal auction deadlines
BidRequest.cur	["USD"]	Explicit currency preference

Changes

File	Change
crates/openrtb/Cargo.toml	New crate manifest for trusted-server-openrtb with serde + serde_json deps
crates/openrtb/src/lib.rs	Complete OpenRTB 2.6 data model: BidRequest, BidResponse, Imp, Banner, Video, Audio, Native, Site, App, Dooh, Device, User, Geo, Regs, SeatBid, Bid, Source, SupplyChain, Pmp, Deal, Publisher, Content, Producer, Network, Channel, EID, UserAgent, Qty, Refresh, DurFloors, and more
Cargo.toml	Add crates/openrtb to workspace members
crates/common/Cargo.toml	Add trusted-server-openrtb dependency
crates/common/src/openrtb.rs	Replace hand-rolled structs with type aliases to new crate. Builder functions enhanced with new params (OpenRtbRequestParams struct, build_publisher, lat/lon/metro on geo, bidfloor/secure/tagid on imp, consent on user, gdpr/us_privacy on regs, dnt/language on device, ref/publisher on site, tmax/cur on request)
crates/common/src/auction/formats.rs	Update bid/response construction to use new builder functions
crates/common/src/integrations/prebid.rs	Map all new OpenRTB fields from available auction data. Extract DNT, Accept-Language, Referer headers. Forward GDPR consent and floor prices. Fix pre-existing test compilation errors for ext field access
openrtb.md	New reference doc: complete OpenRTB 2.6 schema cheat-sheet

Closes

Closes #386

Test plan

• cargo test --workspace (467 tests pass)
• cargo clippy --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check
• JS tests: cd crates/js/lib && npx vitest run
• JS format: cd crates/js/lib && npm run format
• Docs format: cd docs && npm run format
• WASM build: cargo build --bin trusted-server-fastly --release --target wasm32-wasip1
• Manual testing via fastly compute serve

Checklist

• Changes follow CLAUDE.md conventions
• No unwrap() in production code — uses expect("should ...") where needed
• Uses tracing macros (not println!)
• New code has tests
• No secrets or credentials committed