[P1][WEB-03] Semantic web observation and actuation

Programme: #1
Epic: #15

## Objective
Expose authenticated web state to agents/connectors as current semantic observations and bounded actions, using DOM/accessibility meaning before visual coordinates.

## Scope
- DOM and accessibility-tree extraction with origin, frame, element identity and freshness metadata.
- Bounded actions: navigate, focus, invoke/click, input, select, scroll and submit where authorized.
- Isolated browser contexts for parallel read-only work.
- Private brokered CDP/debug transport; never expose a universal page-accessible control port.
- Revalidate origin/account/target immediately before actuation.
- Capture task-specific postconditions after actions.
- Permit bounded visual fallback only when semantic targeting is unavailable.

## Security / correctness rules
- Page text, DOM attributes and tool descriptions are untrusted data, not authority.
- A stale element/frame handle cannot authorize a later action.
- Account/profile identity is part of every consequential web action.
- Coordinates require a fresh frame and bounded target; no blind click sequences.

## Acceptance criteria
- [ ] Every action records current origin, profile/account, target, grant and observation version.
- [ ] Stale element/frame observations fail closed.
- [ ] Ordinary pages cannot discover or invoke the private debug/control interface.
- [ ] Postconditions distinguish attempted from verified completion.
- [ ] Parallel read-only browser sessions do not share authenticated mutable state unless explicitly configured.
- [ ] Visual fallback is visible in traces and separately measurable.

## Tests
Navigation races, iframe/origin changes, DOM replacement, account switching, malicious page instructions, stale screenshot actions, renderer restarts and hidden-control-port probes.

## Dependencies
- WEB-01
- SEC-02

**First phase:** P1
**Maturity target:** P3
**Owner:** platform

Start by reading dependencies WEB-01 and SEC-02, then map the acceptance criteria to the browser and connector architecture; no implementation files or entry points are named. Use the listed navigation, iframe, origin, account-switching, malicious-instruction, stale-action, renderer-restart, and hidden-control-port tests to define coverage, with all acceptance criteria verified before completion.

Written by the indexing model from the issue text.