test: stress supervisor-owned runtime streams

[skevetter]

The owned supervisor has process-cleanup coverage, but its real gRPC transport has not been stressed with large duplex streams. Add an internal fixture and supervisor-backed probes that transfer 100 MiB of stdin and verify 100 MiB on both stdout and stderr, distinct binary tails, and exactly one terminal exit.

The probes exercise paced host/plugin consumers, paused input and output backpressure, cancellation, synthetic command early exit, and an actual plugin process crash while stdin is active. Sender readiness is acknowledged, and sender termination is checked before cleanup can mask an early-exit/crash hang. Incremental hashes keep the test payload out of retained memory. A separate control stream measures live Go heap after GC while data is paused, enforces a 32 MiB growth budget over the connected baseline, and verifies the transport remains usable after cancellation.

This is transport evidence, not a peak-RSS bound or certification of a real runtime's process/environment behavior. Runtime executable paths include spaces and Unicode. The existing Linux/macOS/Windows race-test jobs and runtime-tests artifacts already include the new probes; no workflow or protocol changes are needed. Real-runtime compatibility and supervisor startup measurements remain separate follow-ups.

Validation:

• Full local race suite and vet passed.
• Strict lint, formatting, pre-commit, and diff checks passed.
• Windows fixture cross-build passed.
• Native CI passed on Linux, macOS, and Windows: race tests, vet, startup probes, generated bindings, Lint, and Pre-commit. Uploaded reports confirm all seven new stress cases passed on each OS.
• Local CodeRabbit completed a review of all five files with zero findings.
• GitHub verified the signed commit.
• Greptile was requested after CI passed; its final confidence/review gate is pending. Hosted CodeRabbit automatic review was skipped because this PR is draft.
• Release Please is intentionally skipped on pull-request runs.

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Reviewing files that changed from the base of the PR and between 2df05ac and 7e2d434.

📒 Files selected for processing (5)

• README.md
• internal/streamfixture/main.go
• internal/streamprobe/fixture_test.go
• internal/streamprobe/stress_test.go
• internal/streamprobe/traffic_test.go

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[skevetter]

@greptileai review