Implement navigation history stack #158

Description#

Implement a session history stack that tracks visited pages and allows back/forward traversal. This is the internal data structure that powers both the History API and the browser chrome back/forward buttons.

Requirements#

Session History#

• Maintain an ordered list of session history entries
• Each entry stores: URL, document state (or ability to reconstruct), scroll position (x, y), serialized state data (for pushState), document title
• Track the current entry index (position in the list)
• Navigating to a new page pushes a new entry after the current position (discarding any forward entries)
• Maximum history size limit (e.g., 50 entries) to prevent unbounded memory growth

Traversal#

• traverse_back() — move to previous entry, reload document if needed
• traverse_forward() — move to next entry, reload document if needed
• traverse_to(delta) — move by delta steps (negative = back, positive = forward)
• Same-document navigations (pushState, fragment changes) don't require full page reload on traversal

Integration Points#

• Link click navigation should push a new history entry
• location.replace() should replace (not push) the current entry
• Form submission should push a new history entry
• Fragment navigation should push a new history entry

Acceptance Criteria#

• Navigating to a new page creates a new history entry
• Back traversal returns to the previous page
• Forward traversal returns to the page you came back from
• Navigating from the middle of the stack discards forward entries
• Scroll position is saved and restored on traversal
• History size is bounded
• Unit tests for stack operations (push, traverse back/forward, truncation)