cmd: add LLGo-aware LLDB launcher

[cpunion]

Summary

Stock LLDB no longer has a maintained Go language plugin, and upstream requires
an RFC plus a committed long-term maintainer before restoring one. Keep the
LLGo-specific integration in this repository:

• add llgo lldb, which embeds and loads the LLGo Python adapter without
  depending on a source checkout;
• support LLDB 18+ auto-detection, LLGO_LLDB, -lldb, and LLDB argument
  pass-through;
• keep one plugin source and make the existing shell launcher a compatibility
  wrapper;
• keep adapter commands under the llgo namespace so stock LLDB p and v
  remain unchanged;
• keep the flat acceptance fixture under cmd/llgo/lldbtest, with source-line
  markers and explicit Python SB API assertions instead of comment parsing;
• run the complete LLDB acceptance suite through llgo lldb on the primary
  macOS and Ubuntu jobs.

This does not change LLGo's current DWARF contract: compile units remain
DW_LANG_C for stock-LLDB variable visibility, while the LLGo producer and
versioned marker select the external adapter. The test suite also verifies that
an ordinary C binary is not identified as LLGo.

Testing

• macOS arm64, Apple LLDB 1703: 198/198 real breakpoint, scope, type, and value
  checks
• Ubuntu amd64, LLDB 19: 198/198 checks
• Linux arm64, LLDB 19.1.7: 198/198 checks
• cmd/internal/lldb statement coverage: 83.9% on both platforms
• go test -vet=off -timeout 30m ./...
• Linux: go test ./cmd/llgo and go build ./...

Refs #2154. Follow-ups #2206 and #2211.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[cpunion]

Superseded by #2211, which retains the launcher and acceptance-suite commits and adds the versioned marker/schema contract. Keeping one standalone PR avoids an unnecessary merge dependency.