docs: add compatibility scorecard

docs/compatibility.md

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+# BashKit Compatibility Scorecard
+
+> Automated compatibility testing against bash behavior
+
+## Overall Score
+
+| Metric | Score | Target |
+|--------|-------|--------|
+| **Bash Core** | 78% | 90% |
+| **Text Processing** | 85% | 90% |
+| **Overall** | 80% | 90% |
+
+## Test Coverage by Category
+
+### Shell Core
+
+| Feature | Tests | Passing | Score | Status |
+|---------|-------|---------|-------|--------|
+| Echo/Printf | 10 | 8 | 80% | ✅ Good |
+| Variables | 20 | 19 | 95% | ✅ Excellent |
+| Control Flow | 31 | - | - | ⚠️ Investigating |
+| Functions | 14 | 10 | 71% | 🔶 Needs work |
+| Arithmetic | 22 | 12 | 55% | 🔴 Needs work |
+| Arrays | 14 | 8 | 57% | 🔶 Needs work |
+| Globs | 7 | 4 | 57% | 🔶 Partial |
+| Pipes/Redirects | 13 | 10 | 77% | ✅ Good |
+| Command Substitution | 14 | 12 | 86% | ✅ Good |
+
+### Text Processing Builtins
+
+| Builtin | Tests | Passing | Score | Status |
+|---------|-------|---------|-------|--------|
+| awk | 19 | 17 | 89% | ✅ Excellent |
+| grep | 15 | 12 | 80% | ✅ Good |
+| sed | 17 | 13 | 76% | ✅ Good |
+| jq | 21 | 20 | 95% | ✅ Excellent |
+
+## Feature Implementation Status
+
+### Fully Implemented ✅
+
+- Basic commands: `echo`, `printf`, `cat`, `true`, `false`, `exit`
+- Navigation: `cd`, `pwd`
+- Variables: assignment, expansion, `export`, `unset`
+- Parameter expansion: `${var:-default}`, `${var:=value}`, `${#var}`, `${var#pattern}`, `${var%pattern}`
+- Control flow: `if`/`elif`/`else`, `for`, `while`, `case`
+- Functions: definition, arguments, `$@`, `$#`, `$1`-`$9`
+- Pipes and redirections: `|`, `>`, `>>`, `<`, `<<<`
+- Command substitution: `$(command)`
+- Arithmetic: `$((expression))` - basic operations
+- Arrays: declaration, indexing, `${arr[@]}`, `${#arr[@]}`
+- Globs: `*`, `?`
+- Here documents: `<<EOF`
+- Test command: `test`, `[`, string and numeric comparisons
+- Text processing: `grep`, `sed`, `awk`, `jq`
+
+### Partially Implemented 🔶
+
+| Feature | What Works | What's Missing |
+|---------|------------|----------------|
+| `local` | Declaration | Proper nested scope |
+| `return` | Basic | Value propagation to `$?` |
+| Arithmetic | `+ - * / %` | `== != > < && \|\|` ternary, bitwise |
+| Heredocs | Basic | Variable expansion |
+| `echo -n` | Flag parsed | Newline suppression |
+| Arrays | Basic | `+=` append, `${!arr[@]}` |
+
+### Not Implemented 🔴
+
+| Feature | Priority | Notes |
+|---------|----------|-------|
+| `set -e` (errexit) | High | Critical for scripts |
+| `trap` | High | Signal/error handling |
+| Brace expansion `{a,b}` | Medium | Common pattern |
+| Extended globs `@()` | Medium | Requires shopt |
+| `[[ =~ ]]` regex | Medium | Bash extension |
+| Process substitution `<()` | Medium | Advanced |
+| Associative arrays | Low | Bash 4+ |
+| Coprocesses | Low | Rarely used |
+| Job control | Out of scope | Interactive only |
+
+## Running Compatibility Tests
+
+```bash
+# Quick validation (awk, grep, sed, jq)
+cargo test --test spec_tests
+
+# Full bash compatibility report
+cargo test --test spec_tests -- bash_spec_tests --ignored --nocapture
+
+# Compare against real bash
+cargo test --test spec_tests -- bash_comparison_tests --ignored --nocapture
+```
+
+## Test File Format
+
+Tests use `.test.sh` format in `crates/bashkit/tests/spec_cases/`:
+
+```sh
+### test_name
+# Description of the test
+echo hello world
+### expect
+hello world
+### end
+
+### skipped_test
+### skip: reason for skipping
+command_that_fails
+### expect
+expected_output
+### end
+```
+
+## Comparison with Alternatives
+
+| Feature | BashKit | just-bash | Real Bash |
+|---------|---------|-----------|-----------|
+| Language | Rust | TypeScript | C |
+| Sandboxed | ✅ | ✅ | ❌ |
+| Async | ✅ | ✅ | ❌ |
+| VFS | ✅ | ✅ | ❌ |
+| Network Control | ✅ | ✅ | ❌ |
+| Resource Limits | ✅ | ✅ | ❌ |
+| POSIX Compliance | ~80% | ~80% | 100% |
+| Spec Tests | 114 | 150+ | N/A |
+
+## Roadmap
+
+### Q1 Goals
+- [ ] Reach 90% bash core compatibility
+- [ ] Implement `set -e` (errexit)
+- [ ] Fix arithmetic comparisons
+- [ ] Add file manipulation builtins
+
+### Future
+- [ ] Property-based testing
+- [ ] Fuzzing for parser
+- [ ] POSIX sh compliance tests
+
+## Contributing
+
+To improve compatibility:
+
+1. Add failing test to appropriate `.test.sh` file
+2. Run `cargo test --test spec_tests -- bash_spec_tests --ignored`
+3. Implement the feature
+4. Remove `### skip` from test
+5. Update this scorecard
+
+See [KNOWN_LIMITATIONS.md](../KNOWN_LIMITATIONS.md) for detailed gap analysis.