Fix native Space scrolling in focus oracle

[chubes4]

Summary

• distinguish Chromium's native Space page-down behavior from application-caused focused-element concealment
• suppress browser-focused-element-hidden only when bounded transition evidence proves an uncanceled generated document Space action retained focus on the same initially visible link and the element geometry moved exactly with the viewport
• retain serious findings for focus loss, inert/hidden content, application movement, explicit script scrolling, and genuinely offscreen targets

Browser Semantic

On a focused normal link, Chromium does not activate the link for Space. It scrolls the document down by roughly one viewport while retaining focus, which moves the link's viewport-relative box offscreen. Buttons instead activate on Space without page scrolling, and links activate on Enter.

Suppression Predicate

The collector records bounded key-transition context only for canonical generated document keyboard actions ending in Space. Suppression requires all of the following:

• canonical generated action identity with no descriptor or selector target
• completed, uncanceled Space key dispatch with no synchronous application scroll
• a visible, rendered, non-inert link at Space keydown
• the same active element after stabilization
• positive vertical viewport movement with no horizontal movement
• unchanged target dimensions and target box movement equal and opposite to the viewport scroll delta
• a still-rendered, non-inert target after the action

A bounded scan diagnostic records the action ID, normalized locator, scroll delta, and active-element continuity. Finding fingerprints remain unchanged because suppression context is not fingerprint input. Replay uses the same action instrumentation and predicate, including unrecorded minimization scans.

False-Negative Safeguards

• Enter link activation and button Space activation do not enter the suppression path.
• Synchronous script scrolling marks the transition as application-produced.
• Opacity concealment, geometry changes, inert transitions, and focus loss fail the predicate and remain serious findings.
• Existing dialog focus, tab order, focus restoration, inaccessible target, capability, and artifact-bound behavior remains covered by the focused and adversarial suites.

Tests

Passing:

• npm run build
• npm run test:browser-accessibility-oracles (32 tests)
• npm run test:adversarial-runtime (16 tests)
• npm run test:redaction
• npm run test:runtime-command-artifact-bounds
• npm run test:schema-parity
• npm run test:runtime-contract-package-exports

Coverage includes desktop 1280x900 and mobile 390x844 Tab+Space behavior, replay, Enter activation, button Space activation, application concealment, inert focus loss, explicit script scrolling, and genuinely offscreen focus.

Unrelated Baselines

• npm run test:public-api-contract fails because the expected dist module list omits the already-exported browser-accessibility.js; this PR does not modify the export or contract test files.
• npm run check passes production-boundary enforcement and the initial smoke commands, then stops at the pre-existing command-registry-smoke assertion that wordpress.collect-workload-result should mention an output schema ID; this PR does not modify command registry files.

Fixes #2068