[maleadt]

The signposts and os_log messages from ObjectiveC.OS never showed up in Instruments or log show. Unified logging doesn't copy a message's name and format strings into the log. It records their offsets in the Mach-O image passed as dso, plus that image's UUID, and the decoder reads the strings back from the image's __TEXT segment on disk. We passed a heap copy of libjulia's file contents as dso, so libtrace dropped every message.

This PR makes those strings static, and makes signposts cheap enough for hot paths like kernel launches in Metal.jl.

Static strings. The name and format strings are emitted as private unnamed_addr LLVM constants (via llvmcall) in a @noinline function specialized on them. When that specialization is compiled into a package image, the constants end up in the image's __TEXT,__cstring section, which the decoder reads like Apple's own __oslogstring (pkgimages drop custom sections, so we can't use that one). A weak hidden __dso_handle reference, which the linker resolves to the image's Mach-O header, provides the matching dso. @signpost_interval and @signpost_event precompile that specialization when they are expanded during precompilation, so call sites that are only compiled at run time still find their strings in the image of the package that contains them. When there is no image (a name that isn't a string literal, or code outside a package image), signposts are named "Julia" with the actual name prepended to the message. If ObjectiveC.jl itself isn't precompiled, they borrow a %s from libobjc, whose strings are always decodable.

Dynamic tracing. OS.CATEGORY_DYNAMIC_TRACING (and CATEGORY_DYNAMIC_STACK_TRACING, CATEGORY_POINTS_OF_INTEREST) are Apple's categories for signposts that stay disabled until a tool asks for them. A disabled @signpost_interval then costs one os_signpost_enabled check and builds no message: about 16 ns on an M1, the same as Apple's C macros. Enabled intervals cost about 250 ns, also the same as C, and allocate nothing. Instruments only enables these categories for subsystems that an instrument names, so OS.instruments_package(subsystems) builds such an instrument with Xcode's instrumentbuilder. It plots the intervals and events of those subsystems on a timeline, lists them, and summarizes the intervals by subsystem and name. Use it with xctrace record --package $path --instrument "Julia Signposts", or open the package in Instruments.app.

Alignment with Apple's macros. Events now use OS_SIGNPOST_ID_EXCLUSIVE instead of the null ID, and signposts with a null or invalid ID are dropped, as os_signpost_emit_with_type does. The enabled checks are declared as returning Bool, matching the C prototypes. The docs now say that OSLog construction (os_log_create, 40–60 ns) should be hoisted out of hot paths, and fix the documented default stop message.

API changes are additive. interval_begin/interval_end/os_signpost_emit_with_type gained methods that take the static strings, and their name and message arguments are no longer restricted to String. The now-unused Libdl dependency is dropped, and the version is bumped to 7.1.0 so that Metal.jl can depend on the release.

Verified with xctrace record on macOS 27 (M1): without this PR the os-signpost table contains no Julia signposts at all. With it, a precompiled package emitting DynamicTracing intervals and events in three subsystems, recorded with a package built for two of them, shows exactly the intervals and events of those two, with their names and messages. The tests check that the strings of a call site in another package end up in that package's image. The mechanism also works in juliac executables and libraries (with and without --trim=safe): the strings land in the binary's __cstring, and log show decodes them.