[docs] milestones m15 rubric: precursor edits (R-4/R-5/R-8)

[trilamsr]

Summary

Three build-approach-independent rubric refinements for M15, surfaced by the research doc (PR #92) and locked-in by RFC-0010 (PR #94). Five additional rubric edits (R-1, R-2, R-3, R-6, R-7) are intentionally withheld per the rationale below.

Diff

3 lines changed in MILESTONES.md:

• R-4 (cosmetic): max_log_size citation now references the OTel container stanza operator default. 1 MiB matches; documents the prior art whether we depend on it (BA-1) or port it (BA-2 per RFC-0010).
• R-5 (new reliability caveat): containerd #11149 silently drops bytes from 0.log when an in-container process reads its own FD 1. Mechanism is shared-pipe contention, not generic backpressure (round-1 research framed it incorrectly; the 2025-01-22 reproducer in the issue pinpoints the mechanism). Standard workloads unaffected.
• R-8 (degraded-mode specificity): rotation-stalled is now defined concretely as 0.log size > containerLogMaxSize for ≥30 s (3× kubelet default containerLogMonitorInterval of 10 s, cited at source). Surfaced via IncError("rotation_stalled"). Prior text was generic "kubelet rotation breakage" with no detection mechanism.

Why not the other 5 rubric edits

• R-1 / R-2 (namespace): OD-12 effectively resolved by the upstream proposal at docs/proposals/gen-ai-training-semconv.md (PR [docs] m13 industry-alignment pass: RFC-0009 + gen_ai.training.* proposal + FOLLOWUPS #93 closed the O4-overdue first-draft KPI). No rename needed.
• R-3 / R-7 (rotation correctness, gzip handling): deferred pending the corresponding TestContainerStdout_* integration-test fixtures landing in the M15 implementation phase per RFC-0010.
• R-6 (bbolt cursor): dropped because BA-2 keeps the JSON cursor at /var/lib/tracecore/container_stdout/cursor.json as originally rubricked.

Test plan

• make doc-check clean (273 markdown links resolve, banned-phrase lint clean across 67 files, RUNBOOK ↔ alerts pairing clean).
• Containerd #11149 mechanism re-verified at the issue's 2025-01-22 reproducer comment; mechanism is shared-pipe contention.
• containerLogMonitorInterval default cited verbatim at pkg/kubelet/apis/config/v1beta1/defaults.go; verified in research §13.8.

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