fix: repair operator timer arming, close the remaining Rx gaps, and cut dispatch cost

[ChrisPulman]

What kind of change does this PR introduce?

Backward-compatible API additions, a scheduling correction across four operators, and two
performance changes, addressing gaps found while investigating #167 and #154. Existing public
APIs remain available.

What is the new behavior?

Rx compatibility APIs

• CombineLatest returns named tuples for 2 to 16 sources without a result selector, in both the
  lean and Reactive packages. It also combines a source collection or a params array into an
  IList<T>, with an optional selector over that list. The tuple overloads outrank the list one,
  so two to sixteen same-typed sources listed inline keep binding to the tuple that names them.
• Signal.FromEventPattern accepts a handler conversion, and a sibling keeps the sender's static
  type through a new EventPattern<TSender, TEventArgs>. Signal.FromEvent covers events that
  carry only their argument, with and without a conversion. Every event bridge takes an optional
  trailing sequencer that attaches and detaches the handler as scheduled work and cancels a
  pending attach on disposal. Supplying a conversion also avoids deriving the handler
  reflectively, which keeps those overloads trim- and AOT-safe.
• Five SubscribePrimitives overloads and one SubscribeSafePrimitives overload explicitly
  select the Primitives implementation when System.Reactive is also in scope. Those are exactly
  the shapes System.Reactive declares in the System namespace, so they are the only ones an
  implicit using System; makes ambiguous.

Scheduling correction

Timer ownership is installed before scheduling, through a shared TimerSlot.Arm. A sequencer that
runs a callback before its own Schedule returns can no longer have the successor timer that
callback armed cancelled when the outer call returns. This covers Calm/Throttle,
Shift/Delay, Every/Interval and Timer; under that interleaving Delay previously
dropped every value and the two timers stopped after a single tick.

Performance

• Combine-latest holds each source's latest value in a slot of that source's own element type
  rather than a shared object array, so a notification no longer boxes. Each slot observes its
  source directly, so a subscription costs one object per source instead of a closure and a
  delegate per callback. This is the arity 3 to 16 path, shared by CombineLatest and
  SyncLatest and by the tuple and list overloads built on them.
• Signal<T>.OnNext no longer takes the observer gate. The single observer, the single action and
  the slot array collapse into one field that a dispatch reads once, with terminal state carried
  in that same field, so a stopped signal stays silent and a disposed one still throws. Subscribe,
  remove and the terminal transitions still take the lock and still reuse the slot array in place.

What is the current behavior?

Tuple- and list-returning combination overloads, custom event-handler conversion, typed-sender
event patterns and sequencer-scheduled handler attachment are all missing. Importing
System.Reactive alongside Primitives makes Subscribe and the observer-taking SubscribeSafe
ambiguous. SubscribeSafe(Action<Exception>) handles terminal errors rather than exception
values, so it is not a safe drop-in for ThrownExceptions.

An operator that arms its next timer from inside the current one loses that successor when the
scheduling call returns. Combine-latest boxes every value for three or more sources, and every
signal notification takes a lock.

Checklist

• I have read the Contribute guide
• Tests have been added or updated (for bug fixes / features)
• Docs have been added or updated (for bug fixes / features)
• Changes target the main branch
• PR title follows Conventional Commits

Additional information

Related to #167 and #154. This PR does not claim to reproduce or resolve the original Skia crash
or the reported early-emission symptom. Existing ambiguous Subscribe calls still require a
consumer-side choice, such as SubscribePrimitives, an explicit static call, or a
namespace-scoped import: overload priority does not resolve extension methods declared in
different classes.

Measured with BenchmarkDotNet on net10.0, short job. Before and after are separate runs on the
same machine; the System.Reactive column comes from the same run as the after column.

Benchmark	Before	After	System.Reactive
Signal emit, 1024 values	6,438 ns	2,091 ns	1,690 ns
CombineLatest, 4 sources, 1000 notifications	20.7 us / 26,400 B	9.3 us / 1,392 B	11.2 us / 976 B
CombineLatest, 2 sources, 1000 notifications	13.6 us / 1,040 B	8.0 us / 1,024 B	9.9 us / 472 B

R3's subject emits 1024 values in 2,274 ns on the same run, so the signal is now ahead of it and
within 24% of System.Reactive, from 3.8x behind. Subscribe and dispose was not traded away for
that: 8 subscribers 241 ns / 696 B and 64 subscribers 3,492 ns / 4,352 B, both ahead of
System.Reactive on time and on allocation.

Validation:

• Full solution build in Release with public API tracking on: zero warnings, zero errors.
• Full test run across every target framework that builds on the host: 12,534 passed, 0 failed,
  0 skipped.
• Two benchmark classes are new, covering combine-latest across the pairwise and array-backed
  coordinators and the timer slot at one arm per window, which the existing burst cases never
  reach.
• Apple target framework baselines cannot be generated on Linux. They were rebuilt from the
  matching non-platform baseline plus their platform-only API and verified line for line; the
  macOS build leg is what confirms them.

[codecov]

Codecov Report

❌ Patch coverage is 99.73822% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.17%. Comparing base (b0fa198) to head (08e6cb8).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...rc/ReactiveUI.Primitives.Core/Signals/Signal{T}.cs	97.64%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #177      +/-   ##
==========================================
+ Coverage   98.13%   98.17%   +0.04%     
==========================================
  Files         705      711       +6     
  Lines       21761    22080     +319     
  Branches     2680     2704      +24     
==========================================
+ Hits        21356    21678     +322     
+ Misses        199      198       -1     
+ Partials      206      204       -2     

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
99.9% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud