No single-file HTML output with inlined assets

[xroche]

HTTrack's only one-file output was -%M, a multipart/related MHT archive. Firefox and Safari never read those, so you cannot hand someone an .mht and count on it opening. --single-file (-%Z) produces ordinary HTML instead: once the mirror completes, every saved page is rewritten in place with its stylesheets, scripts, images and fonts embedded as data: URIs. Links between pages stay relative, so the mirror is still a browsable tree and any single page can also be handed to someone on its own. The assets do end up on disk twice. -%M stays, and the two are good at different things: MIME carries text parts without the base64 tax, keeps each resource's original URL and stores a shared asset once, while single-file HTML wins on opening by double-click anywhere. The help text and the guide say which to reach for; neither presents one as the successor.

It cannot reuse the MHT path. postprocess_file() streams a MIME part per file as it is saved, but a data: URI needs the asset's bytes at the moment the page is written, and pages are normally saved before their assets are fetched. So this is a pass over the finished tree, hooked at the tail of httpmirror() after the update purge. By then the engine has made every reference relative, so resolving one is path arithmetic against the page's own directory, clamped to the mirror root: the HTML is hostile input and a crafted ../../.. must not reach a file outside the project. A reference carrying a scheme is skipped, which covers data: and makes a second --update run a no-op rather than a double-encode. Audio and video keep their links, as do page-to-page links and anything over --single-file-max-size (10 MB by default, reported in the log). htsopt.h gains two tail-appended fields; I have not touched VERSION_INFO.

An inlined stylesheet becomes a data: URL, and per the URL Standard a data: URL has an opaque path. So a relative reference left inside one does not resolve: the parser returns failure, whatever path we write there. That only bites an asset the stylesheet referenced and the cap kept as a link, and raising --single-file-max-size past it inlines the asset too. htssinglefile.h and the docs record the limitation.

-#test=singlefile builds a fixture mirror and asserts what is inlined, what keeps its link, the cap, the per-page inline budget, the mirror-root clamp and idempotence. tests/93 drives a real crawl, decodes the emitted payloads with Python and compares them byte for byte against the served files, checks the rewritten page keeps the mode the engine gave it, and checks the webhttrack form still emits the right command line. The negative-case fixtures plant a real file wherever deleting a guard would send the resolver, so no assertion can pass just because the target was missing; I deleted each guard in turn and watched the suite go red. A libFuzzer harness covers the rewriter, since it re-serializes attacker-controlled HTML.

The option is wired into the webhttrack Build tab, all 30 locales, the help text, a regenerated man page and the command-line guide.

Closes #713

[xroche]

Two notes for anyone reading the commit list rather than the diff.

The branch's first commit still says Single-file output is MHT, which browsers no longer open. That framing is wrong and was corrected once #713 was retitled: MHT is not going anywhere, -%M stays supported, and the two are siblings rather than successor and predecessor. The PR title and body carry the right framing, and since this repo squash-merges, they are what becomes the commit message, so the stale subject never reaches master. I left it rather than force-push over a branch that already has CI history against it.

Separately, #726 tracks something this PR noticed but deliberately did not fix: replace_file() in htsback.c and wacz_rename_over() in htswarc.c were already two copies of the unlink-then-rename fallback, and singlefile_rewrite_file() makes three, with two different return conventions between them. Refactoring two shared files inside a feature PR is review risk for no functional gain, so it wants its own change.