Improve signal interruption handling and stale lock recovery

[nhan7777]

Summary

Addresses user-reported issue where interrupting flutter_pruner scan with Ctrl+C during long operations leaves a stale lock that blocks subsequent runs with unclear "corrupt" error messages.

Changes

1. Progress Feedback

• File: lib/src/cli/commands/scan_command.dart:336-339
• Change: Added "(may take 30-60s on large projects)" suffix to adapter analysis progress messages
• Impact: Users now understand that long operations are expected, reducing impulse to interrupt

2. Improved Error Messages

• File: lib/src/core/project/project_operation_lock.dart:73-100
• Change: Distinguished stale armed records from actual corruption with specific recovery guidance
• Impact: When a stale lock is detected, users get:
  • Clear explanation: "operation was interrupted and did not complete cleanup"
  • Step-by-step recovery instructions with exact commands (ps aux | grep flutter_pruner → rm → retry)
  • Distinct from genuine corruption cases that require deeper inspection
• Test: Added test/core/project_operation_lock_test.dart:676-730 verifying stale armed detection logic

3. Documentation

• File: doc/signal-handling.md (new)
• Content: Comprehensive guide covering signal handling behavior, long-running operation expectations, stale lock recovery procedures, and fail-closed lock design rationale
• Links: Added reference in README Safety section and CHANGELOG

Root Cause

When user sends SIGINT/SIGTERM during long-running dart analyze (~30s on large projects), if the Flutter Pruner process receives a second signal (or SIGKILL) during the 5-second termination cleanup window, the process terminates before writing unconfirmed/cleared records → lock file contains only armed record → next run fails with "corrupt" error.

Design Decisions

• Fail-closed design maintained: Armed records without process identities remain authoritative blockers requiring manual verification before removal (cannot auto-clear due to PID reuse risks)
• No second-signal blocking: User pressing Ctrl+C twice signals clear intent to force quit
• Manual recovery acceptable: Clear guidance is better than unsafe auto-cleanup

Verification

• ✅ Manual testing: Triggered stale lock, verified improved error message displays correct recovery steps
• ✅ Test suite: 13/13 lock tests pass including new stale armed detection test
• ✅ Targeted verification: dart analyze clean on modified files
• ✅ Documentation: Comprehensive signal handling guide added

Related Issues

Fixes user-reported "Flutter Pruner process uncertainty evidence is corrupt" error when interrupting scans.