feat: implement quote processing (Phase 6 M13)

Milestone 13 complete: POSIX-compliant quote processing with
quote-aware glob expansion integration.

## New Module

- src/quotes.rs (~600 lines):
  - QuoteState enum: Unquoted, SingleQuoted, DoubleQuoted
  - QuotedSegment struct: text + quote state pairing
  - parse_quotes(): Full POSIX quote parser
  - should_expand_glob(): Quote-aware glob pattern detection
  - reconstruct_string(): Segment reconstruction helper
  - 17 unit tests (all passing)

## Parser Integration

- Modified src/parser.rs:
  - quoted_word_to_string(): Now escapes glob metacharacters (*, ?, [, {)
    in quoted contexts (both single and double quotes)
  - Escaped patterns prevent glob expansion via contains_glob_pattern()
  - Preserves existing variable expansion handling

## POSIX Quote Semantics

- Single quotes ('...'):
  - All characters literal, no expansion whatsoever
  - Cannot contain single quote (even escaped)
  - Whitespace preserved

- Double quotes ("..."):
  - Variable expansion: $VAR, ${VAR}
  - Command substitution: $(cmd), `cmd`
  - Arithmetic expansion: $((expr))
  - NO glob expansion (patterns stay literal)
  - Backslash escapes: \$, \`, \", \\, \newline

- Backslash (unquoted):
  - Escapes next character literally
  - Line continuation: backslash-newline removed

## Quote + Glob Integration

- Quoted globs don't expand:
  - '*.txt' → stays literal "*.txt"
  - "*.txt" → stays literal "*.txt"

- Unquoted globs expand:
  - *.txt → file1.txt file2.txt

- Escaped globs don't expand:
  - \*.txt → stays literal "*.txt"

## Testing

- All 157 library tests passing (including 17 quote tests)
- Integration verified: escaped glob patterns not detected
- Quote behaviors: whitespace, escaping, line continuation

## State Updates

- STATE.scm:
  - Overall completion: 85% → 90%
  - Phase: M12 Complete → M13 Complete
  - Removed "Quote processing not implemented" blocker
  - Added quote processing + quote-aware glob to working-features
  - Added session history entry

## Documentation

- docs/PHASE6_M13_DESIGN.md: Complete design document (800+ lines)

Phase 6 progress: 13/14 milestones complete (90%)
Next: M14 Full POSIX shell compliance (final milestone)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: Test

STATE.scm

@@ -26,8 +26,8 @@
      (version . "0.14.0"))
 
     (current-position
-     (phase . "Phase 6 M12 Complete - Glob Expansion Operational")
-     (overall-completion . 85)
+     (phase . "Phase 6 M13 Complete - Quote Processing Integrated")
+     (overall-completion . 90)
 
      (components
       (proofs
@@ -73,6 +73,8 @@
       "Here documents (<<DELIMITER with expansion and <<-DELIMITER with tab stripping)"
       "Here strings (<<<word for single-line input)"
       "Glob expansion (*.txt, file?.rs, [a-z]*, {1,2,3}, brace expansion)"
+      "Quote processing (single quotes, double quotes, backslash escaping)"
+      "Quote-aware glob expansion (quoted patterns don't expand)"
       "Redirection undo support (file truncate/append reversible)"
       "Pipeline undo support (final redirections reversible)"
       "SIGINT handling (Ctrl+C interrupts commands/pipelines, not shell)"
@@ -193,8 +195,7 @@
      (critical
       "Formal Lean → Rust correspondence proofs still needed (mechanized verification)")
 
-     (high
-      "Quote processing not implemented (Phase 6 M13)")
+     (high)
 
      (medium
       "Full POSIX compliance (subset) - Phase 6 M14"
@@ -224,6 +225,35 @@
       "Begin Lean 4 → Rust correspondence proofs"))
 
     (session-history
+     ((timestamp . "2026-01-29")
+      (session . "phase6-m13-quote-processing")
+      (accomplishments
+       "Completed Phase 6 Milestone 13: Quote Processing"
+       "Created comprehensive quote module (src/quotes.rs, ~600 lines):"
+       "  - QuoteState enum: Unquoted, SingleQuoted, DoubleQuoted"
+       "  - QuotedSegment struct: content + state pairing"
+       "  - parse_quotes(): POSIX-compliant quote parser"
+       "  - should_expand_glob(): Quote-aware glob detection"
+       "  - reconstruct_string(): Segment reconstruction"
+       "  - 17 unit tests covering all quote behaviors"
+       "Integrated with existing parser quote handling:"
+       "  - Modified quoted_word_to_string() to escape glob metacharacters in quotes"
+       "  - Escapes *, ?, [, { in single and double quotes"
+       "  - Escaped patterns prevent glob expansion (via contains_glob_pattern())"
+       "  - Preserves whitespace in quoted strings"
+       "POSIX-compliant quote semantics:"
+       "  - Single quotes: all characters literal, no expansion"
+       "  - Double quotes: variable/command expansion, no glob expansion"
+       "  - Backslash escaping: unquoted and in double quotes"
+       "  - Line continuation: backslash-newline removed"
+       "Quote + Glob integration:"
+       "  - Quoted globs don't expand: '*.txt', \"*.txt\" stay literal"
+       "  - Unquoted globs expand: *.txt → file1.txt, file2.txt"
+       "  - Escaped globs don't expand: \\*.txt stays literal"
+       "All 157 library tests passing (including 17 quote tests)"
+       "Integration test verified: escaped globs not detected as patterns"
+       "Phase 6 progress: 13/14 milestones complete (90%)"
+       "Next: M14 Full POSIX compliance (final milestone)"))
      ((timestamp . "2026-01-29")
       (session . "phase6-m12-glob-expansion")
       (accomplishments