Hold a rewrite recording per flow instead of per thread

[Rafael-SOWNet]

Closes #859.

RewriteRecording held its ambient scope in a [ThreadStatic] field, and documented the consequence rather than fixing it:

A synchronous scope, and it has to be. Do not await inside one.

It no longer has to be. The scope is an AsyncLocal, as MathS.Settings now is (#857) and as the cancellation token in MathS.Multithreading already was.

Why this is more than a field swap

The trap #857 ran into applies directly here. An AsyncLocal flows the reference, so once the pointer reaches child tasks, two of them can report to one recording at the same time — and the steps were accumulating into a List<RewriteStep>. That is a torn write, not a merged list.

So alongside the field:

• the store is a ConcurrentQueue<RewriteStep>;
• closed is volatile, since it is now read from flows other than the one that set it;
• Steps copies out rather than handing back a live view.

Behaviour

	was	is
a recording across an await	lost, and the thread could collect a stranger's rewrites	kept
work started under a recording, on another thread	not collected	collected
two flows each with their own recording	separate	separate
a recording opened inside a task, seen after it ends	no	no

One existing test is rewritten, not deleted

ARecordingOnOneThreadDoesNotSeeAnother started a thread inside an open recording and asserted its work was not collected. ExecutionContext flows to a manually started thread, so that work is now collected — which is the point of the change rather than a regression of it.

It becomes WorkStartedUnderARecordingIsCollectedWhereverItRuns, and the isolation it was really reaching for — that a parallel caller records its own work and nobody else's — is covered properly by SiblingRecordingsDoNotSeeEachOther, with a barrier forcing both recordings open before either does any work.

Also new: ARecordingSurvivesAnAwait (the regression) and ARecordingOpenedInsideATaskDoesNotEscapeIt.

AClosedRecordingIgnoresWhateverItIsStillHanded passes unchanged.

Off is still free

RewriteAllocationTest passes untouched: no recording open still costs one ambient read per rule set and allocates nothing, which is what #746 asks of every layer above the tree.

Verification

• 6064 C# tests pass, 0 failed
• 130 F# wrapper tests pass

Breaking, so it wants the 2.0 window; recorded in BREAKING-CHANGES.md with the migration, the Steps-is-now-a-snapshot note, and the fact that step order across parallel work is not defined.

Note on merge order: this touches BREAKING-CHANGES.md at the same anchors as #853 and #857. Whichever merges later needs a trivial "keep both" resolution in that file.

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