Add session-owned Chrome viewport emulation for responsive FSV

[ChrisRoyse]

Parent: #717. Related: #1202, #1212.

Problem

During calyxwebsite #1261 layout FSV, Synapse could open background tabs in the existing Chrome window and capture target-specific screenshots, but there is no exposed tool to set a session-owned tab viewport / device emulation for phone-width visual verification. (Confirmed absent: no Emulation.setDeviceMetricsOverride path in crates/synapse-mcp/src.)

Related friction: after cdp_navigate_tab on a session-owned Chrome bridge target, target_act verb=screenshot refused the same target with ACTION_TARGET_INVALID / missing session-owned Chrome bridge owner row. Opening a fresh session-owned tab restored capture.

Needed capability

• Session-targeted tool to set viewport bounds / device emulation (width/height/deviceScaleFactor, mobile flag) on an existing Chrome tab without launching a new browser — via CDP Emulation.setDeviceMetricsOverride.
• Separate SoT readback of viewport width/height/deviceScaleFactor after setting it.
• Target screenshot ownership survives same-session cdp_navigate_tab, or cdp_navigate_tab refreshes the owner row so target_act verb=screenshot stays valid.

FSV context

• Existing Chrome HWND 2427400.
• Screenshot worked on a fresh tab, producing chrome_debugger_page_capture_screenshot_bgra captures.
• Desktop proof possible; phone-width proof was limited to built-CSS/source checks because viewport emulation is not exposed.

[ChrisRoyse]

Carry-forward directive - remove foreground-protection limiting, preserve 50+ foreground-capable lanes

This issue inherits the current operator direction recorded on #1223: #1223 (comment)

• Do not create, continue, revive, merge, depend on, or reintroduce foreground protection, background-only, broad foreground-denial, or capability-suppressing limiter policy/implementation under another name.
• Rework any code, docs, profile, tool description, prompt, policy, branch/worktree state, or product behavior that strands valid foreground or foreground-equivalent agent work.
• Target behavior is 50+ concurrent agents: use background/target-scoped routes when sufficient, and foreground-equivalent lanes/routes whenever valid work requires foreground semantics.
• Keep human_os_foreground separate from each agent's agent_logical_foreground / foreground_lane / owned target. Raw real OS foreground is a leased singleton break-glass path only, not the concurrency mechanism.
• Tool exposure is system behavior. Steer agents to intent-level routers and lane-aware routes without suppressing reachable foreground-equivalent capability.
• Browser work must use the already-open authenticated Windows Chrome session; do not launch new Chrome browser processes for browser work.
• Synapse behavior acceptance must use real wired Synapse MCP tool calls plus separate physical Source-of-Truth readback when a tool surface exists. Supporting scripts/tests/checks/builds are regression evidence only, never FSV.
• In this single-agent repo run, stale claimed, status:in-progress, or old agent labels are historical/resumable state, not another active Synapse repo agent.
• Issue-close hygiene remains mandatory: implementation commit/push before close when code changed, evidence/readback before close, then close-marker commit/comment. Commits include [skip ci].

Context-only comment; implementation and manual FSV remain on the issue itself.