docs(calls): call stability hardening spec and phased plan

[webguru-hypha]

Summary

• Adds call-stability-hardening-spec.md — normative CSH-* requirements for Human panel call reliability (sidebar/dock contract, screen share handoff, mesh scale bounds, multi-tab sync, recording stability, SFU upgrade path).
• Adds call-stability-hardening-plan.md — phased implementation plan (P0–P6) with spec ID traceability and QA checkpoints.
• Links Phase 7 from voice-video-call-implementation-plan.md to the new hardening docs.

This is spec-only — no runtime code changes. Intended as the source of truth for follow-up implementation PRs after hotfix #2284 merges.

Test plan

• Review spec §2 scale tiers and CSH-* requirement IDs for completeness
• Confirm plan phases P0–P6 align with spec sections and PR fix(calls): restore in-chat call banner after PR #2276 regression #2284 scope
• Verify cross-links resolve (call-stability-hardening-spec.md, call-stability-hardening-plan.md, voice-video-call-implementation-plan.md)

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📥 Commits

Reviewing files that changed from the base of the PR and between 081b0ed and 06dda6d.

📒 Files selected for processing (3)

• docs/requirements/call-stability-hardening-plan.md
• docs/requirements/call-stability-hardening-spec.md
• docs/requirements/voice-video-call-implementation-plan.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch spec/call-stability-hardening

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[webguru-hypha]

Superseded by #2297, which merges the CSH stability spec/plan together with the WCUX world-class spec and adds call-consolidated-implementation-plan.md for a single implementation PR. Safe to close this PR once #2297 is merged.