Runtime async outlining prototype - #20746

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Continue outlining past bindings that cannot be outlined, copy arguments
duplicated into match targets, restrict sequential handling to NormalSeq,
guard the outline walk against deep let chains, and share the Await
builder and match/quotation helpers.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Outline suspending InlineIfLambda callbacks and prepare runtime-async bodies in a dedicated pass after the first optimization loop, so every body is prepared once in its final shape.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Outlined callbacks ran as separate runtime-async methods, so AsyncLocal writes they made were discarded on return. A callback selected by if/match is now constructed once, recording its branch and captures in locals, and each invocation dispatches to an inlined copy of the selected body.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Every invocation copies every branch body. Skip the rewrite when the copies would exceed 2000 expression nodes, leaving the callback as a closure so a remaining suspension is reported as FS3918. Add regression tests for callbacks constructed in a loop and nested branch-selected callbacks.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

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