Complete HTML elements and attributes coverage (WHATWG living standard)

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This PR brings TyXML up to date with the WHATWG HTML living standard. All changes are backward compatible, except two additions to the Wrapped_functions module type (noted below).

New elements

• s, bdi, search (flow grouping content), data (with a mandatory ~value argument, following the img precedent)
• track: void element with a mandatory ~src argument, plus its attributes a_kind, a_track_srclang (scoped rename of srclang for track) and a_default. audio and video gain an optional ?tracks argument mirroring ?srcs, and the PPX assembler partitions track children accordingly.
• slot: transparent content model, encoded like canvas/map

selectedcontent is deliberately left out: typing the customizable select content model would weaken button typing globally while the spec is still evolving; it remains reachable through Unsafe.

New attributes

• Global: popover (a bare popover parses as auto in the PPX), inert, enterkeyhint, autocapitalize, autocorrect, writingsuggestions, nonce, slot, is, and `Auto for a_dir
• Microdata: itemscope, itemtype, itemid, itemprop, itemref
• CSS Shadow Parts: part, exportparts
• Buttons: popovertarget, popovertargetaction, and the Invoker Commands command/commandfor (custom --* commands via `Other, following the linktype design)
• loading, decoding, fetchpriority (img; also link/script where specified), srcdoc and allow (iframe), as, imagesrcset, imagesizes, blocking, nomodule (link/script/style), ping (a/area), dirname and capture (form controls), type on ol (a_ol_type, case-sensitive values with a dedicated PPX parser), abbr on th, closedby on dialog, media on meta, playsinline, disablepictureinpicture, disableremoteplayback, and the declarative shadow DOM attributes on template (shadowrootmode etc.)
• Some membership fixes for existing attributes where the spec allows them (crossorigin/usemap/referrerpolicy on img, referrerpolicy on a/area, autocomplete on select/textarea, disabled on link)

New event handler attributes

• Pointer events, onwheel and onauxclick, typed with the existing Xml.mouse_event_handler (PointerEvent and WheelEvent inherit from MouseEvent in the DOM; adding dedicated handler types to Xml_sigs.T would break every Xml implementation)
• 14 missing global handlers: onbeforeinput, onbeforematch, onbeforetoggle, oncancel, oncontextlost, oncontextrestored, oncopy, oncut, onpaste, oncuechange, onscrollend, onsecuritypolicyviolation, onslotchange, ontoggle
• Window handlers on body: onlanguagechange, onrejectionhandled, onunhandledrejection

Compatibility notes

• The Wrapped_functions module type gains string_of_blocking and string_of_ol_type. Implementations that delegate to Html_f.Wrapped_functions (e.g. js_of_ocaml-tyxml) are unaffected; hand-written ones need two delegation lines. This is called out in CHANGES.md.
• Thanks to the signature reflection mechanism, the PPX and JSX pick up almost everything automatically; only four custom value parsers were added (popover, command, ol_type, blocking), each tested through both the [%html] and JSX front-ends.

Each addition comes with rendering tests (test_html.ml) and PPX round-trip tests (test_ppx.ml), a CHANGES.md entry, an MDN link in the documentation, and the spec link in its commit message.

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Note on CI: master's CI has been red for a while because recent compilers turn warning 67 (unused functor parameter) into an error in the dune development profile; the failures initially reported on this PR were exactly those pre-existing errors, not caused by these changes. The fix is in #366's sibling PR #367; this branch has been rebased on top of it so its CI can run green. Once #367 is merged, this branch can be rebased onto master and the extra commit will disappear from the diff.

The remaining (ubuntu-latest, 4.08) failure is unrelated infrastructure: the opam switch initialisation itself fails to solve ocaml-base-compiler for 4.08 with opam 2.5.2 (it fails on master too, before any tyxml code is built).