Fix multi ranges

[5cript]

Fix multi ranges - PR changelog

Summary

Observed<T> now lets multiple consumers subscribe to the same container without stepping on each other. Previously, RangeEventContext was a producer-side singleton on the Observed, so two Nui::range(vec) subscribers shared one diff state and each consume-and-reset raced the other: only the first subscriber to render saw the update.

This PR moves RangeEventContext to be per-consumer, owned by each BasicObservedRenderer, and has the producer broadcast change events to all live readers.

Example:

#include <nui/frontend.hpp>

using namespace Nui;
using namespace Nui::Elements;
using namespace Nui::Attributes;

void example()
{
    Observed<std::vector<std::string>> items{{"alpha", "beta", "gamma"}};

    auto renderItem = [](long long, auto const& s) -> ElementRenderer {
        return li{}(s);
    };

    // Two views over the same Observed:
    //   - a sidebar list
    //   - a footer count + repeated list
    //
    // Before this PR, only the first range() to render would receive
    // updates; the second silently fell behind because both shared the
    // Observed's single internal RangeEventContext.
    //
    // After this PR, both stay in sync.
    return body{}(
        div{class_ = "sidebar"}(
            ul{}(range(items), renderItem)
        ),
        div{class_ = "footer"}(
            span{}([&items]() { return std::to_string(items.size()) + " items"; }),
            ul{}(range(items), renderItem)   // <-- same Observed, second subscriber
        )
    );

    // items.push_back("delta");           // both lists grow
    // items[0] = "alpha-renamed";         // both lists update at index 0
    // items.erase(items.begin() + 1);     // both lists shrink
}

Breaking changes

• ObservedContainer::rangeContext() (both overloads) removed — the range event context is now an implementation detail of range rendering, not a property of the observed value.
• ObservedContainer constructors taking RangeEventContext&& (4 overloads) removed.

These were unused outside the test suite; if you somehow held one, replace with the default constructor.
Because of that I consider this change not major version increasing, because its likely 0 impact.

Behavior changes

• Multiple Nui::range(observed) subscribers over the same Observed<vector|deque|string|set|list> now all stay in sync across mutations. Previously the second-and-later subscribers would silently fall behind.
• Observed<set> and Observed<list> automatically opt their attached reader contexts into disableOptimizations=true, since their non-random-access iteration model can't replay the optimized diff path.

New API

• ObservedContainer::attachReaderContext(std::shared_ptr<RangeEventContext> const&): called by range renderers during attachment.
• ObservedContainer::readerContextCount(): raw size of the reader vector (debug/test helper).
• RangeEventContext::setDisableOptimizations(bool): replaces the removed boolean constructor.

Tests

• ~24 new MultiSubscriber_* tests covering every mutating op (push/pop front+back, emplace, insert ×4 overloads, erase ×2, resize ×3, indexed-assign, iterator-deref-assign, swap, assign, full reassignment, clear+repopulate).
• 8 lifecycle/stress tests: SubscriberLifetime_Detach, SubscriberLifetime_DiesMidBroadcast, FiveSubscribers_RandomMutations, InteractionAcrossSubscribers, OperatorAssignBetweenContainers, Map_Set_Deque_MultiSubscriber, MoveAssignedObservedHasNoSubscribers, RepeatedIndexedAssignAtSamePosition.

Test mock fixes

• replaceWith now actually replaces self's entry in the parent's children / childNodes arrays (was previously redirecting self's shared ReferenceType, which only worked for the first replace at any DOM position; subsequent replaces became invisible to the parent).
• replaceWith's no-parent fallback (used for root setBody) also rebinds document.body to the new value when self is the current body, so chained body replacements within a single test no longer leave document.body pointing through a stale reference chain.

Dependencies

• Bumped interval-tree to the latest pinned commit.

Test results

405 passing, 0 disabled