feat(Async): StartTaskImmediate - #1

…InlineIfLambda (dotnet#20368)

* Perf: eliminate per-call closure in StackGuard.Guard via InlineIfLambda

StackGuard.Guard was called at 18 hot compiler sites as
`guard.Guard <| fun () -> body`, allocating a heap closure for the
`fun () -> body` on EVERY call even though the common path just runs
`f ()` inline. This was the #1 allocator in the compiler
(remapExprImpl guard closure alone ~312 MB/compile of a 65k-LOC input).

Make Guard an `inline` member with `[<InlineIfLambda>] f`, so the fast
path inlines the body (zero closure). The rare stack-insufficient path
(metrics + async SwitchToNewThread) moves to a non-inline
`RunOnNewStack` member so the heavy code isn't duplicated at call sites.
Depth accounting moves to non-inline `EnterGuard`/`ExitGuard` members so
the inlined body touches no private field (realsig-safe; no FS1113).

Since the `<|` operator defeats InlineIfLambda, the 12 `<|` call sites
are converted to direct-call `Guard(fun () -> ...)` syntax; the 4
existing direct-call sites benefit unchanged.

Result on a 65k-LOC compile (Release, net11): total allocation
10630.7 -> 9979.0 MB/compile (~651 MB, 6.1% saved); the guard closures
remapExprImpl@1749 (312 MB), accFreeInExprNonLinear@1099 (105 MB),
OptimizeExpr@2519 (59 MB), accExpr@45 (40 MB), TransExpr@1130 (20 MB)
all drop to 0. Emitted output is byte-identical (IL disassembly 0 diffs;
single-file outputs hash-identical). Deep-recursion behavior preserved:
200000-deep guarded recursion still jumps threads (73 jumps) and
completes with no Field/MethodAccessException under --realsig+.
IL size delta +85.5 KB (+0.41%).

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

* Add regression test: inline Guard common path allocates no closure

EmittedIL/Inlining test with a vanilla StackGuard emulation (inline Guard +
[<InlineIfLambda>] + separate non-inline RunOnNewStack the lambda escapes into).
Asserts the direct-call common path allocates 0 bytes/call, while the same lambda
through `<|` allocates every call. Guards against InlineIfLambda no longer
eliminating the per-call closure, and documents why the call sites use direct
application rather than `<|`.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 4862d2e9-7740-4d0b-ba96-113e99466711

---------

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Co-authored-by: perf-bundle <perf@local>
Copilot-Session: 4862d2e9-7740-4d0b-ba96-113e99466711