Replace ProcessInfoLayoutRenderer with ProcessStartLayoutRenderer - #6224

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@src/NLog/LayoutRenderers/ProcessInfoLayoutRenderer.cs`:
- Around line 173-174: The managed memory value in
ProcessInfoLayoutRenderer.ProcessInfoLayoutRenderer is being truncated by the
unchecked `(int)GC.GetTotalMemory(false)` cast, which can overflow for heaps
above 2 GB. Update the logic that computes and formats `currentMemorySize` to
clamp the `GC.GetTotalMemory(false)` result instead of directly casting it to
`int`, so `AppendFormattedValue` receives a safe IntPtr-sized value without
negative overflow.
- Around line 150-164: The 64-bit branches in
ProcessInfoLayoutRenderer.ProcessInfoLayoutRenderer for
ProcessInfoProperty.MaxWorkingSet/NonPagedSystemMemorySize/PagedSystemMemorySize
and the *64 variants are using unchecked((int)long.MaxValue), which truncates to
-1. Update the first group’s 64-bit path to emit int.MaxValue as the sentinel,
and update the PagedSystemMemorySize64/NonPagedSystemMemorySize64 64-bit path to
emit long.MaxValue while keeping the 32-bit path as (long)int.MaxValue.

In `@src/NLog/LayoutRenderers/ProcessInfoProperty.cs`:
- Line 41: The obsolete message on ProcessInfoProperty should point users to the
actual supported alias. Update the Obsolete attribute text in
ProcessInfoProperty so it references ${processstart} instead of
${processstarttime}, keeping the rest of the guidance aligned with the existing
processid, processname, and gc aliases.

In `@src/NLog/LayoutRenderers/ProcessStartLayoutRenderer.cs`:
- Around line 88-91: `ProcessStartLayoutRenderer.GetValue()` is still returning
the `LogEventInfo.ZeroDate` sentinel, so `${processstart}` never reflects the
actual process launch time. Update the `ProcessStartLayoutRenderer`
implementation to read and return a real process-start timestamp from the
runtime/process API instead of `ZeroDate`, and keep the existing `UniversalTime`
handling by converting that real start time to UTC or local as needed.

---

Duplicate comments:
In `@src/NLog/LayoutRenderers/ProcessInfoLayoutRenderer.cs`:
- Around line 44-53: The obsolete guidance for ProcessInfoLayoutRenderer is
inconsistent between the XML summary and the [Obsolete] attribute. Update the
ProcessInfoLayoutRenderer documentation and attribute message so they list the
same alternatives, and make sure the Obsolete text matches the summary wording
around ${processid}, ${processname}, ${processstarttime}, ${processtime}, and
${gc}. Also review the related ProcessInfoProperty reference so the
${processstarttime} mention is consistent there as well.

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@src/NLog/LayoutRenderers/ProcessInfoLayoutRenderer.cs`:
- Around line 47-53: Update the obsolete guidance in ProcessInfoLayoutRenderer
so the summary, remarks, and [Obsolete] message point users to ${processstart}
instead of ${processstarttime}; keep the existing references to ${processid},
${processname}, ${processtime}, and ${gc}, and ensure the wording matches the
new renderer name introduced by the PR.

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Nitpick comments:
In `@src/NLog/LayoutRenderers/GarbageCollectorInfoLayoutRenderer.cs`:
- Around line 61-65: The garbage collector info renderer duplicates the same
invariant numeric append logic that already exists in
StringBuilderExt.AppendNumericInvariant. Update
GarbageCollectorInfoLayoutRenderer to use the shared helper for the value
formatting path instead of keeping the framework-conditional cast and
AppendInvariant logic inline, so the Int64/UInt64 handling stays aligned with
the existing implementation.

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@src/NLog/LayoutRenderers/GarbageCollectorInfoLayoutRenderer.cs`:
- Around line 61-66: The GC info formatter currently only uses invariant
formatting under NETFRAMEWORK, while the non-Framework long path in
GarbageCollectorInfoLayoutRenderer.Append still falls back to culture-sensitive
formatting. Update the Append logic so the long-value path also uses
builder.AppendInvariant(value) (or equivalent invariant handling) instead of
builder.Append(value), keeping the formatting consistent across all targets and
locales.