tp: implement perfetto_metadata clock/machine overrides

[LalitMaganti]

Add the perfetto_metadata sidecar: a JSON file inside a zip/tar
archive which overrides clock and machine handling for the other files
in the archive, together with a comprehensive diff test suite covering
every green and error path of the v1 schema.

The schema (v1):

• detection via the top-level perfetto_metadata key; own trace type,
  processed before all other archive members (including proto)
• top-level trace_time_clock (builtin clock names)
• per-file entries matched by exact path with two section objects:
  machine ({id}) and clocks (native, offset_ns XOR anchor)
• anchors are expressed as 'a ts as written in the file' (file-native
  units, e.g. us for JSON) paired with a value on a named builtin
  clock or a utc wall time; the source clock is always implicit
• strict validation: hard errors for unknown fields, unknown clock
  names, unmatched/duplicated paths, mutually exclusive fields,
  standalone (non-archive) usage and multiple config files

The implementation:

• kPerfettoMetadataTraceType is parsed before everything else in the
  archive. The file does NOT fork a per-trace context: it produces no
  events, and forking would create the per-machine state early and
  steal 'primary' status from the real traces, isolating their clock
  snapshots in private syncs.
• PerfettoMetadataReader strictly parses/validates the config into the
  new global TraceMetadataState. trace_time_clock is applied directly
  on TraceTimeState with a sentinel owner id so no later trace file
  can override it.
• ForwardingTraceParser::Init consults the entry matching the file's
  archive path: machine.id feeds ForkContextForTrace, clocks.native
  remaps the format's declared clock (new ClockTracker remap consulted
  in ToTraceTime), clocks.offset_ns becomes a per-file post-conversion
  shift and clocks.anchor is injected as a snapshot into the machine's
  shared ClockSynchronizer so it composes with snapshots from other
  traces. For proto traces a clocks override also routes packets
  without an explicit clock id through the trace default clock.
• Overrides only apply to files which are single-clock/single-machine
  in practice; proto traces are checked optimistically and fail lazily
  (a ClockSnapshot or a packet with machine_id != 0 proves multi-ness).
  Entries whose path matches no archive member fail at end of file.
• Fix SystraceTraceParser to write the clock-converted timestamp back
  into SystraceLine: previously the conversion was only used as the
  sorting key while tables were populated from the raw MONOTONIC
  value. This was masked by identity conversions and only diverged
  when a proto trace's snapshot actually linked MONOTONIC into the
  clock graph.

[github-actions]

🎨 Perfetto UI Builds

• cfd688d (2026-06-10 12:30 UTC): https://storage.googleapis.com/perfetto-ci-artifacts/gh-27275788503-1-ui/ui/index.html
• 7b79c57 (2026-06-10 12:40 UTC): https://storage.googleapis.com/perfetto-ci-artifacts/gh-27276389789-1-ui/ui/index.html
• 9f9ca13 (2026-06-10 14:13 UTC): https://storage.googleapis.com/perfetto-ci-artifacts/gh-27281847401-1-ui/ui/index.html
• 02e351b (2026-06-11 16:39 UTC): https://storage.googleapis.com/perfetto-ci-artifacts/gh-27361956112-1-ui/ui/index.html