src: run same-priority platform tasks in posting order

[codebytere]

TaskQueue<T> has been a std::priority_queue since #58047 so that worker tasks honor v8::TaskPriority. Its comparator returns false for entry types without a priority member and for equal priorities, and the comment expects that to keep insertion order. A binary heap doesn't: three tasks pushed A, B, C pop as A, C, B, and 64 pushes come back out as 0, 2, 6, 14, 30, 62, ...

Three queues are affected: the per-isolate foreground queue (tasks of one priority no longer run in the order they were posted), the foreground delayed queue, and the worker runner's DelayedTaskScheduler, where it can hang shutdown. That scheduler drains its local queue in one batch, so when V8 posts a delayed worker task shortly before the platform shuts down, the StopTask pushed by Stop() can run before a ScheduleTask that was pushed earlier; the ScheduleTask then starts a timer on the scheduler's loop after all timers were stopped, and Shutdown() sits in uv_thread_join() until the delay expires (8 s when the late task is the memory reducer). It shows up as an intermittent multi-second exit stall in processes that exit soon after doing some work.

This is the other half of the shutdown race #61999 addressed: that change makes PostDelayedTask() return early once Stop() has run (both under the tasks_ lock), so every ScheduleTask that is queued was queued before the StopTask; with posting order restored it also runs before it, and no further flag is needed.

The fix gives every queued item a sequence number and uses it as the tie breaker, so equal-priority and priority-less tasks come out FIFO again while higher priorities still go first; PopAll() returns the tasks in that order instead of handing out the heap, which also removes the const_cast loops at its three call sites.

Tests: new cctest (TaskQueueTest.HigherPriorityFirstThenPostingOrder: FIFO across PopAll()/Pop() for a priority-less queue, priority-then-FIFO for TaskQueueEntry) fails on main and passes here; cctest and the default suite pass.

Refs: #58047
Refs: #61999

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Disclosure: the code, test and this description were written by Claude Code, directed and reviewed by @codebytere.

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.12%. Comparing base (30bff4a) to head (25b1017).
⚠️ Report is 36 commits behind head on main.

Files with missing lines	Patch %	Lines
src/node_platform.cc	94.73%	0 Missing and 1 partial ⚠️
src/node_platform.h	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65353      +/-   ##
==========================================
- Coverage   90.13%   90.12%   -0.02%     
==========================================
  Files         752      752              
  Lines      251568   251857     +289     
  Branches    47270    47364      +94     
==========================================
+ Hits       226759   226991     +232     
- Misses      16168    16191      +23     
- Partials     8641     8675      +34     

Files with missing lines	Coverage Δ
src/node_platform.cc	74.75% <94.73%> (-1.25%)	⬇️
src/node_platform.h	85.71% <80.00%> (-5.96%)	⬇️

... and 55 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/75958/