test: pass robot documents in memory and drop empty trybuild globs

[jBernavaPrah]

Purpose

Completes phase 3 of the accepted test-boundary refactor.

What changes

• The robot document reaches generation through generate's override seam instead of a temporary file: the schema-rejection and minimal-surface rules are now tested against the document itself, in memory. Loading a real file reaches the same validation through every scaffolded project in the build_api integration suite.
• The trybuild compiler contracts keep one fail case per distinct guarantee — a survey found no redundant permutations to slim, so nothing was deleted from real coverage. What was empty is gone instead: trybuild_host_ui matched no fixtures at all (its host_pass/host_fail directories never existed), and trybuild_ui's pass glob matched nothing. Both dead paths are removed; the module doc now names where successful authored usage is actually proven — runtime_authoring and runtime_semantics, 106 deterministic tests that compile and run the real macro surfaces without invoking the compiler again.

No deeper in-memory seams were added for phoxal-build: the two semantic tests that needed no filesystem no longer touch it, and the remaining generation tests legitimately need prepared products and directories — new seam machinery would serve no current test.

Verified

• phoxal-build lib: 18/18.
• trybuild: 5 fail cases in 26 s warm; scenario suite (4 fail, 1 pass) in 63 s warm.
• clippy --workspace --all-targets -D warnings, fmt --check clean.

Delivery order

Single repository, no publication impact.