Support per-section GAM ad unit paths via gam_unit_path templates

[prk-Jr]

Summary

• creative_opportunities.slot.gam_unit_path is now a template with {network_id}, {section}, and {slot_id} placeholders, so a publisher whose GAM ad unit varies by site section expresses that in one slot rule instead of one rule per (slot × section).
• {section} is derived from the request path (first segment, sanitized), with a required section_root for / — the URL→section convention lives in config, not core.
• Server-only change: gam_unit_path reaches googletag.defineSlot client-side but is not in the OpenRTB request, so the wire shape is unchanged and there is no JS change.

Changes

File	Change
crates/trusted-server-core/src/creative_opportunities.rs	Add UnitTemplatePart, parse_unit_template, sanitize_section, derive_section; slot compiled_unit cache + compile_unit_template/render_gam_unit_path/template_uses_section; config section_root + compile_unit_templates; validate_runtime now enforces section_root when {section} is used and drops the render-time emptiness check
crates/trusted-server-core/src/publisher.rs	build_slot_json/build_ad_slots_script take request_path; render the template on both the initial-render and handle_page_bids (SPA) paths
crates/trusted-server-core/src/settings.rs	prepare_runtime parses templates via compile_unit_templates before validation, surfacing parse errors as Configuration errors
docs/guide/configuration.md	New "Creative Opportunities Configuration" section documenting templating, {section} derivation, the no-decode rule, and unmatched-route behavior
trusted-server.example.toml	Document section_root and a templated-slot example
docs/superpowers/specs/, docs/superpowers/plans/	Design spec and implementation plan

Closes

Closes #954

Test plan

• cargo test-fastly && cargo test-axum (also test-cloudflare, test-spin — all pass)
• cargo fmt --all -- --check
• cargo clippy-fastly (remaining clippy targets left to CI)
• Docs format: cd docs && npm run format
• JS tests / JS format — no JS change in this PR; left to CI
• clippy-axum / clippy-cloudflare* / clippy-spin* — left to CI

New tests: template parsing (unknown placeholder, unmatched/nested brace, empty), section derivation (/, single/multi-segment, unsafe/undecoded segment new%20s → new_20s), render_gam_unit_path (template, default, verbatim), startup validation (missing/invalid section_root, parse error), and initial-render/SPA equivalence for the same path.

Hardening note

Template parsing and section_root validation are config-derived and run at startup in Settings::prepare_runtime. Invalid enabled config fails startup via Report<TrustedServerError::Configuration> — no panic!, unwrap(), or expect() on the config path. compile_unit_templates rejects malformed templates (unknown placeholder, unmatched/nested brace, empty); validate_runtime rejects a {section} template with a missing or non-[A-Za-z0-9_-] section_root. Regression coverage: compile_unit_templates_surfaces_parse_error, validate_runtime_requires_section_root_when_template_uses_section, validate_runtime_rejects_invalid_section_root, and settings_rejects_creative_opportunity_slot_with_empty_gam_unit_path. Existing static gam_unit_path configs are unaffected (verbatim when no placeholders; /{network_id}/{slot_id} when absent).

Checklist

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