feat: ship fenced multi-tab inference broker alpha

[aantenore]

Objective

Deliver a provider-neutral same-origin browser broker with one fenced inference owner, peer streaming, bounded admission, takeover, and an executable visual lab.

Contract impact

• Requirement IDs: R1-R10
• Protocol or configuration change: introduces protocol v1, monotonic ownership epochs, and validated broker configuration
• Privacy or security change: telemetry is payload-free; same-origin trust boundary and residual risks are documented

Evidence

• Format, lint, and type checks
• 57 unit tests with coverage thresholds
• Library and demo builds
• Package checks and clean-consumer smoke
• 12 browser coordination tests across Chromium, Firefox, and WebKit
• Production dependency audit
• Visual verification at desktop and mobile widths

Residual risk

Real WebLLM and Transformers.js adapters remain unverified integration seams. Provider execution is at-least-once across takeover. One low-severity development-only esbuild advisory remains mitigated by loopback binding while the supported Vite line catches up.

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