refactor(conformance): re-home misplaced logic across tooling seams

[e54-bot]

Summary

Closes #50 — gives each conformance-tooling module a single caller-shaped concern:

• scenario.rs → project_fixture.rs: the session-runtime machinery (ProjectFixture, materialize_project, substitute_project_uri) moves to a new sibling module consumed only by session. scenario.rs is back to being exactly what its doc says: fixture schema + structural validation (+ envelope defaults and session-template splicing). 297 → 249 LOC.
• Step's either/or is unrepresentable: Step deserializes via a private StepWire + TryFrom into StepRequest::{Request, RawLine}. Both/neither request/rawLine now fails at deserialization — with the same error message, plus line/column — so the validator's "exactly one" clause and session.rs's unreachable! are deleted. step_request_line is infallible.
• accepted_artifact_formats moves to methods.rs beside select_artifact_format: ADR-0004's semantics get a single edit site, and Server's surface is back to session state + generic gates (since, capability).

Behavior-preserving: no wire, spec, or fixture changes.

Test plan

• cargo fmt --all --check
• cargo clippy --all-targets -- -D warnings
• cargo test --workspace (14 tests)
• lpp-conformance-runner --validate-only --fixtures conformance/fixtures/v1 — 61 passed
• lpp-conformance-runner --provider ./target/release/lpp-mock-provider --fixtures conformance/fixtures/v1 — 61 passed
• git diff --check
• Manually verified: fixtures with both or neither of request/rawLine still fail validation with step must have exactly one of 'request' or 'rawLine'

[Teakowa]

conformance/runner/src/scenario.rs — StepWire/TryFrom<StepWire> introduces the key behavioral invariant of this refactor, but no automated test exercises either invalid case. The existing conformance fixtures are valid inputs, so both request+rawLine and neither can regress while the full fixture suite stays green. Add focused tests that deserialize a step/scenario and assert both invalid shapes are rejected. Assert the behavior, not incidental serde line/column formatting.

[e54-bot]

Review finding addressed in 171d41d.

Step request-shape invariant now has focused coverage — scenario.rs gained a test module that deserializes Step directly (smallest owning surface): both request+rawLine present and neither present are asserted to fail with the rule's own message (exactly one of 'request' or 'rawLine'), plus a positive case that each valid form parses. Assertions target the rejection behavior, not serde's line/column formatting.

Verification: cargo test --workspace (17 tests, +3), clippy -D warnings, fmt, both conformance runs 61/61, git diff --check.

Ready for follow-up review.

[Teakowa]

LGTM