Add publisher origin Host header override

[ChristianPavilonis]

Summary

• Adds optional publisher.origin_host_header_override configuration for publisher fallback origin requests.
• Registers Fastly dynamic backends with the configured Host override while preserving origin_url for connection, SNI, and certificate checks.
• Documents the TOML/env setting and validates host/host:port values.

Changes

File	Change
crates/trusted-server-core/src/settings.rs	Adds publisher config field, helper, env/default behavior, and validation tests.
crates/trusted-server-core/src/host_header.rs	Adds shared Host header override validation helper and unit tests.
crates/trusted-server-core/src/backend.rs	Adds Host header override support and includes override in backend names to avoid dynamic backend collisions.
crates/trusted-server-core/src/publisher.rs	Uses the configured override for publisher fallback proxy backend registration and outbound Host.
crates/trusted-server-core/build.rs, crates/trusted-server-core/src/lib.rs	Wires the validation helper into build/runtime code.
docs/guide/configuration.md	Documents origin_host_header_override and env var usage.
trusted-server.toml	Adds a commented example setting.

Closes

N/A

Test plan

• cargo test --workspace
• cargo clippy --workspace --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 --package trusted-server-adapter-fastly --release --target wasm32-wasip1
• Manual testing via fastly compute serve
• Other: cd docs && npx prettier --check guide/configuration.md

Note: full cd docs && npm run format currently fails on existing base-branch formatting in superpowers/specs/2026-05-28-edgezero-image-optimizer-primitive-design.md; changed docs file passes targeted Prettier check.

Checklist

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