[TumCucTom]

Summary

Direct backend lowering diagnostics reported a generic
direct backend local type is unsupported message regardless of the
actual target, even though the expected and help fields already
named the target correctly. The fix substitutes the generic
direct backend prefix with a target-specific label
(COFF x64, ELF64, AArch64 Mach-O, x86_64 Mach-O, COFF AArch64,
AArch64 ELF) when the IR lowering message is wrapped into a
buildability diagnostic.

The bug was originally surfaced as part of the agent-surface
repros in #68, where the message stayed target-blind in spite of
the target-correct expected and help strings. The agent-surface
classification for the owned-drop-direct-backend-unsupported
fixture already called out "keep backend diagnostics target-correct"
as a follow-up.

Changes

• Add z_direct_backend_target_label helper in target_backend.c.
• Use it in init_lowering_backend_diag (the message-rewrite site
  hit by the targetReadiness JSON for lowering blockers) and in
  z_build_diag (the z_direct_buildability_check funnel).
• Leave messages that do not start with the direct backend
  prefix untouched so callers that already pass a target-aware
  message are unaffected.
• Bump the file-line budgets in scripts/compiler-metrics.mts to
  cover the new helper and the rewritten branch.
• Add regression assertions in conformance/run.mjs covering
  linux-musl-x64, win32-x64.exe, and darwin-x64 against the
  existing owned-drop-direct-backend-unsupported.0 fixture, so
  a future regression to the generic prefix fails conformance.

Verification

• ./.zero/bin/zero check --json --emit obj --target <tgt> conformance/agent-surface/fixtures/owned-drop-direct-backend-unsupported.0 for linux-musl-x64, linux-x64, linux-musl-arm64, win32-x64.exe, darwin-x64, darwin-arm64 now reports
  direct ELF64 local type is unsupported,
  direct ELF64 local type is unsupported,
  direct AArch64 ELF local type is unsupported,
  direct COFF x64 local type is unsupported,
  direct x86_64 Mach-O local type is unsupported, and
  direct AArch64 Mach-O local type is unsupported respectively.
• node --experimental-strip-types --disable-warning=ExperimentalWarning scripts/compiler-metrics.mts reports zero budget violations.
• ZERO_NATIVE_TEST_ALLOW_LOCAL=1 node conformance/run.mjs reports
  conformance ok, including the new target-aware message assertions.

Out of scope

The same generic prefix is also used in emit_*.c files
(emit_elf64.c, emit_coff.c, emit_macho64.c, ...) that
report CGEN004 from the emit layer. Those code paths are only
hit if buildability passes and emission later fails, which is a
different surface than the agent-surface fixture. They are not
covered here to keep the change scoped to the buildability path
that the user-visible diagnostic flows through today.

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