Let the HTML output follow links like a browser

[dayglojesus]

Fixes #86.

The HTML output view allowed every tm-file: and file: navigation to load in the web view. A Markdown preview links to other files relative to the document, WebKit shows a .md target as plain text, so following a link replaced the preview with the raw file. A fragment link ([terms](#terms)) resolves against the document's <base href>, which is not the preview page's own URL, so it replaced the preview with the raw document itself. Both reproduce on main by clicking the links in the preview of a file like the one in the issue.

The preview now behaves like a browser:

• OakLocalLinkActionForURL (new, Frameworks/HTMLOutput/src/helpers/OakLocalLinkPolicy.mm) decides what to do with a tm-file:/file: navigation: only link clicks are affected; a fragment of the document itself scrolls in place; an existing file whose type conforms to HTML, image or PDF loads in the web view as before; directories and missing paths are left to the scheme handler.
• Any other file is offered to the view's owner through a new localFileHandler hook on OakHTMLOutputView. OakCommandRefresher, which already keeps an auto-refreshing preview up to date, takes the link when its command accepts the target (command_accepts_document: whole-document input and a matching scope selector, new in command/parser), re-runs the command with the target as the document and streams the result into the same view as a new page. Back and forward work through WebKit's history, and refreshes follow the document being shown. When the owner declines, or there is none, the file opens in TextMate through the existing txmt://open handler.
• Running the command again from another document in the same project window steers the existing view to that document instead of opening a second output window (browsingRefresherForCommand: in OakTextView).
• Back and forward: command output is streamed once and its pipe is then spent, so WebKit could only show such a page again from its own page cache; on a miss the scheme handler answered 404 and the pane went blank. OakHTMLOutputPageCache (new) keeps the last 32 pages served and the handler replays them. The view also remembers the environment each page was produced with and restores it when history lands on that page, so fragment and relative links keep working there, and it tells the owner which document is now showing so refreshes follow it.
• Refresh in place: an auto-refresh used loadHTMLString:, which replaces the history entry with a document WebKit cannot revisit, so going back to it after following a link failed with "frame load interrupted", which HOBrowserView swallowed while leaving the progress bar running. A refreshed page is now served again under its own URL through the page cache and reloaded, so its history entry stays revisitable, and an interrupted load resets the status bar.
• OakTxMtOpenURLForPath percent-encodes the path so & and = survive the query parser in handleTxMtURL:.
• Tests: HTMLOutput/tests/t_link_policy.mm covers the decision table with temporary files, the URL encoding and the scroll script; t_page_cache.mm the page cache; command/tests/t_accepts_document.cc covers the acceptance predicate.

Verification on macOS 27.0, Debug build, with the issue's sample file previewed via Show Preview:

• Clicking [glossary](GLOSSARY.md#terms) renders GLOSSARY.md in the same preview window as a new page (the view's URL moves from …/Show Preview/1 to …/Show Preview/2), with no editor window opened and no second preview window. History back returns to the README page. Show Preview from README again re-renders README in the same window. Clicking [guide](./docs/guide.md "Guide") renders docs/guide.md. Clicking [terms](#terms) changes the preview's URL fragment without leaving the page. On main the file links replace the preview with raw Markdown.
• Back and forward through README, GLOSSARY.md and docs/guide.md land on the right page every time over a dozen hops, including after an edit refreshed the README page in place before and after following a link, with the status bar back button and a real mouse click on the link; a fragment link clicked after going back scrolls in place; an edit to README after going back to it refreshes the preview.
• HTMLOutput_tests: 13 tests pass; command_tests (run serially, as CI excludes this suite from ctest): 6 tests pass. With the policy forced to "load" the link policy tests fail.