Complete Bun cli/test scanner and isolation corpus compatibility

Focused follow-up to #66 for the top-level cli/test Bun corpus boundary.

Scope:

• recursively discover .test/.spec files while pruning dot-directories and node_modules
• distinguish relative directory scans from explicit file selection
• preserve child stdout and aggregate pass summaries
• parse --preload <file> without treating the preload as a test
• model --isolate discovery for explicit files and whole directories
• cover leaked watcher/server/timer global collection observables

Acceptance:

• cli/test/bun-test.test.ts: 73 passed, 0 failed, 0 unsupported, 6 upstream todos
• cli/test/isolation.test.ts: 19 passed, 0 failed, 0 unsupported, 0 todos
• complete top-level cli/test: 208 passed, 0 failed, 0 unsupported, 6 upstream todos
• full home_test gate passes

Completed in 48e83eb.

Implemented:

• recursive Bun test discovery with dot-directory and node_modules pruning
• relative directory versus explicit file selection
• spawned test stdout propagation and aggregate pass reporting
• --preload <file> argument consumption
• isolation discovery across explicit files and whole-directory runs
• observable global collection output for leaked watcher/server/timer cases

Verification:

• cli/test/bun-test.test.ts: 73 passed, 0 failed, 0 unsupported, 6 upstream todos
• cli/test/isolation.test.ts: 19 passed, 0 failed, 0 unsupported, 0 todos
• full top-level cli/test: 208 passed, 0 failed, 0 unsupported, 6 upstream todos
• isolated js/bun/http/bun-serve-static.test.ts: 34 passed, 0 failed, 0 unsupported, 0 todos
• clean full home_test gate: exit 0

The first full-gate attempt encountered an OS SIGKILL in the existing static-response stress test while .zig-cache occupied 12 GB and the disk had 3.6 GB free. The stress file passed in isolation, the regenerable cache was cleared, and the clean full gate then passed.