test: consolidate test targets and add a prepare-free unit lane

[jBernavaPrah]

Purpose

Final step of the accepted test-boundary refactor: target layout consolidation and explicit CI suite selection.

What changes

• phoxal's six external test targets become three: tests/unit is one executable with modules for the deterministic public-API suites; runtime_behavior_observers keeps its own process (tracing delivery consults a process-wide call-site interest cache); tests/compile gathers the compiler contracts — macro compile-fail cases and the consumer feature profiles' compilation checks. The trybuild fixture expectations are re-blessed for their new directory — only the recorded fixture paths changed, no diagnostic text.
• A new unit lane in CI selects packages and targets explicitly and needs no fixture preparation and no composition compilation: the library and binary unit suites of the SDK, tool, supervisor, services, and components, plus the SDK's external-crate unit and observer suites. phoxal-service-world stays in the prepared lane because its transport test compiles only against its own prepared products.
• That gate was hiding more than a layout detail: services/world was never prepared in CI, so its transport test has been compiling empty — the same hidden-test problem the refactor set out to remove, in a quieter form. The prepare loop now includes it and the test runs (1.8 s against the real transport).
• The clippy-and-tests lane documents that it also executes the documentation tests, and the README now describes all four lanes.

Verified with the exact lane commands

• Unit lane: 13 package library suites green, cargo-phoxal's 76 bin tests, unit 108, observers 4.
• Compile suite: 7/7, and 8/8 with the scenario feature.
• Workspace lane: 53 test binaries, 0 failures (including the world transport test, 1/1).
• clippy --workspace --all-targets -D warnings, fmt --check, git diff --check clean.

Delivery order

Single repository, no publication impact.