Layout, intrinsic sizing, inline collection and margin collapsing recurse per
nesting level with large frames, so 45 nested divs (18 tables, 28 flex
containers) overflowed a 1.5 MB stack and ended the process. Each of these
walks now checks RuntimeHelpers.TryEnsureSufficientExecutionStack and leaves
the rest of the subtree out instead.

Without the stack guard the test host crashes with a stack overflow.

A trial layout is laid out on its own, so the same inputs give the same
result within one render. Nested tables and inline-blocks trial-laid out
every level again for each level above it, doubling the work per level
(20 nested tables took 16 s, 22 over a minute). Trials inside a trial,
the ones the layout around them repeats, are now kept per element with
their inputs (style map, width, inherited style, measuring mode) for the
render, not inside a line-clamped box; incremental rendering drops them
on every change.

Without the trial cache, 6 nested tables take 63 trial layouts and 12 take
4095 (2^depth - 1).

Laid out only a few times, the layout stays in the first JIT tier, whose
frames are large: 12 nested inline-blocks overflowed the test thread's
stack on macOS. With tiered compilation off, devel and the trial cache
need the same stack.

Each of their levels lays out the levels below it twice, so up to the
depth the stack allows the test never completed in CI. They come back
once the nested trial layouts are reused (#54).

Reuse nested trial layouts within a render

…again

With the nested trial layouts reused (#54), they render in well under a
second.

Leave out deep subtrees instead of overflowing the stack