Lockfile trailing newline fix and normalization for frozen lockfile comparison

[brooke-hamilton]

Summary

Fixes lockfile handling to ensure trailing newlines and normalize formatting when comparing lockfiles in frozen mode.

Fixes #1008

Changes

• Trailing newline: Lockfiles now always end with a trailing newline per POSIX convention
• Frozen lockfile comparison: Normalizes existing lockfile content through JSON.parse → JSON.stringify before comparing, so cosmetic formatting differences (indentation, trailing whitespace, missing newline) don't cause false mismatches in frozen mode
• Added comments: Explanatory comments for the POSIX newline convention and normalization logic

Modified Files

• src/spec-configuration/lockfile.ts - Core lockfile logic with newline and normalization fixes
• src/test/container-features/lockfile.test.ts - New tests for:
  • Lockfile ends with trailing newline
  • Frozen lockfile matches despite formatting differences
  • Frozen lockfile fails when lockfile does not exist
  • Corrupt lockfile causes build error
  • No lockfile flags with no existing lockfile is a no-op
• Test fixture updates to include trailing newlines in expected lockfiles
• New test fixture lockfile-frozen-no-lockfile for missing lockfile test

[chrmarti]

Thanks! Left a question.