Save breakpoints and signal settings in projects ##projects

[trufae]

Projects now preserve software and hardware breakpoints, watchpoints, and debugger signal settings.

• Keep breakpoint names, commands, conditions, expressions, enabled/trace state, and counters. Resolve addresses through the existing module rebasing.
• Reuse existing breakpoints on repeated loads, replace changed types, and avoid applying dbb twice. Skip internal and software-step breakpoints.
• Make breakpoint script/diff output replay correctly with dbg.hwbp and dbb set. Fix dbx assigning an expression to its value instead of the current breakpoint, and free expressions on deletion.
• Preserve explicit signal resets. prj load merges settings; prj open clears previous breakpoint and signal state.
• Write project version 6 while retaining version 5 loading. Validate new records and string references without reading across entry boundaries.

Rebased onto master 8b9fdb7cda, with three commits: breakpoint expression handling, breakpoint persistence, and signal persistence. Removed the unrelated map changes and reduced net implementation growth by 61 lines compared with the original PR.

Validation: full make -j4 build; r2r -u -C test db/cmd/newprj db/cmd/cmd_db: 50 passed, zero failures. Coverage includes round-trips, repeated loads, rebasing, script/diff replay, project opening, version 5, and malformed records.

Live debugger tests could not run in this environment: ptrace-traceme: Operation not permitted. CI remains the check for native debugger execution.