fix(authoring): reduce compatibility scan work and drain concurrent results

[777genius]

Avoids an extra allocation-heavy traversal of opaque installer JSON fields and prevents concurrent command tests from hanging when a worker encounters invalid output.

The compatibility scanner skips opaque root values while preserving historical installer acceptance, root duplicate diagnostics, syntax/trailing rejection and the existing depth ceiling. Authoring still performs its bounded, context-aware structural pass. Concurrent test workers return raw results; the parent drains them before assertions.

Verified at 1ceafe6, based on main 34befab:

• Focused conformance, loader and command regressions passed after integration. Implementation race checks passed.
• The 660 KB regression fixture dropped from 23.48 MB / 840,550 allocations to 2.76 MB / 53. The allocation ceiling fails against the previous implementation. This measures the root-check stage, not total installer memory.
• Fault subprocesses verify malformed output and stderr failures terminate instead of losing a concurrent result.
• Independent xhigh review closed both P2 findings with no new P1/P2: 4,488 differential scanner cases, deterministic author cancellation/budget probes, concurrent barriers and lost-send negative control passed. Final required CI, CodeQL, security, docs and Linux/Windows smoke passed.

Follow-up to two confirmed P2 findings from the post-merge audit of #157. Three files, 284 changed lines. No new installer policy, native acquisition change or public authoring release. Revert independently if needed.

[coderabbitai]

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