Fix spurious BROKEN frames on musl-based Linux stacks

[brianrob]

Why

Traces captured with one-collect on musl-based Linux distros (e.g. Alpine) show a large fraction of stacks marked BROKEN when opened in PerfView or TraceEvent. On a real Alpine trace, about 18% of samples were broken. The root cause is that thread base frames on musl live in the combined libc + dynamic loader module, named like ld-musl-x86_64.so.1, which ReasonableTopFrame did not recognize as a legitimate thread-start module. We already special-case glibc's libc for the same reason; musl was simply missing.

What

• TraceEventStacks.cs: In ReasonableTopFrame, treat any module whose name starts with ld-musl- (case-insensitive, so it covers x86_64, aarch64, etc.) as a valid top frame, mirroring the existing glibc libc handling. When matched, a stack rooted there is no longer wrapped in a BROKEN frame.

• Added Universal/MuslBrokenStackTests.cs: a unit test that synthesizes a tiny in-memory V6 nettrace (no large trace binary added to the repo) containing a musl loader module, an ordinary library, and two cpu samples: one rooted in the musl module and one rooted in the ordinary library. It asserts the musl-rooted stack is not BROKEN while the ordinary-library-rooted stack still is, parameterized over both ld-musl-x86_64.so.1 and ld-musl-aarch64.so.1.

Validation

• A throwaway TraceEvent-based console app run against the real Alpine trace confirmed broken stacks dropped from 18.4% to 3.7% (eliminating ~6,100 musl-rooted frames). The scratch app was not committed.
• Confirmed the new test fails without the fix (musl stack marked BROKEN) and passes with it.
• Full TraceEvent suite is green: 2272 passing on net8.0 and 2289 on net462, no regressions.

Notes for reviewers

The negative-case assertion (ordinary library still BROKEN) is intentional to guard against the match being too broad. The test writer emits a non-zero syncTimeQPC so iterating relative timestamps does not trip a Debug.Assert in QPCTimeToRelMSec; this is a test-harness detail only.