Add explicit startup experience hub

[MajorIncident]

Issue

Implements #330.

Current implementation

Startup detection

• deterministic substantive saved-Intake predicate;
• ignores automatic/presentation-only noise such as saved timestamps, bridge time, theme, Notes open state, table labels, action analysis IDs, and unchecked steps;
• detects saved Intake, Student resume, and Instructor resume independently;
• expired Classroom envelopes are not offered as Continue choices;
• safe #join=<human-code> is detected but not auto-admitted.

Explicit startup hub

• ordinary startup no longer silently resumes a stored experience preference;
• explicit Standalone ?workspace= links remain authoritative and bypass the hub;
• Continue section renders independent cards for saved Intake, Student class, and Instructor class;
• Start / Join section uses Work independently, Join a class, and Run a class;
• Administration / Maintenance remains a secondary privileged entry;
• a join link highlights Join a class and leaves the human code untouched until normal Student routing consumes it;
• opening the hub never clears saved work;
• starting a fresh independent Intake requires confirmation when substantive saved Intake exists;
• choosing a different Student/Instructor experience explicitly confirms replacement of the same-type valid resume;
• expired same-type resumes are cleared only when the user explicitly chooses that new intent.

Resume recovery

• Student/Instructor lifecycle controllers remain authoritative;
• invalid/expired server-backed resumes can return to the required startup hub with a human-facing explanation rather than leaving a dead shell.

Coverage already added/updated

• startup substantive-detection unit tests;
• production-markup startup hub feature tests;
• experience-role tests updated for no implicit preference/#join routing;
• Standalone/Student/Instructor reload E2E now clicks the correct Continue card;
• join-link integration now proves explicit startup Join before admission;
• expired Student E2E proves stale context cannot silently resume.

Still in progress

• run/fix repository quality + required browser gate;
• startup-hub accessibility/mobile/reduced-motion audit;
• README/architecture/security/workstream documentation reconciliation;
• final exact-head validation and production verification.

Cold restart

Continue from feature/startup-experience-hub. Read #330 and docs/preproduction-hardening.md. Preserve the key invariant: startup intent is navigation/session state, not Intake persistence, and multiple resumable contexts must remain separate choices.

[MajorIncident]

Mini milestone — startup browser-gate repair

Head: 3f3cfad3a3b350d90f07a60f662e5d3f7fbdf9cb

Proven before this head:

• repository npm run quality: green;
• Dependency Review: green;
• Template Manifest Guard: green;
• CodeQL: green;
• Classroom feature harnesses now explicitly activate Student/Instructor roles and the unit cascade is resolved.

Browser failure isolated from run #1552:

• 26 browser tests passed, 12 skipped;
• only 4 Standalone journeys failed;
• clicking Work independently stayed data-experience-role="unselected".

Root cause:

• isSubstantiveSavedIntake() compared the entire normalized Major Incident object shape against a synthetic baseline;
• the real blank DOM collector includes empty changeControl.rollbackPlan / verification.result keys, while the synthetic baseline omitted them;
• therefore the app-generated bridge timestamp autosave was falsely classified as substantive and triggered the replacement confirmation; Playwright auto-dismissed it.

Fix on this head:

• Major Incident substantive detection is semantic, not structural;
• default owner/risk/change-control/verification values do not count as work;
• actual decision/risk content still does;
• regression test now models the real blank Major Incident collector shape.

Next action: inspect exact-head CI and fix only concrete remaining validation failures.