fix(bash): backslash-newline line continuation in double quotes (#247)

## Summary
- Fix `\<newline>` inside double-quoted strings to act as line
continuation (discard both chars)
- Fixed in both `read_double_quoted_string` and `read_word`'s inline
quote handler
- Per POSIX, `\<newline>` in double quotes should be removed, joining
the lines

## Test plan
- [x] 5 new spec tests for line continuation in double-quoted strings,
variable assignments, multi-line, unquoted, and escape preservation
- [x] All 1199 spec tests pass (1194 pass, 5 skip)
- [x] `cargo clippy` and `cargo fmt` clean

Co-authored-by: Claude <noreply@anthropic.com>

Author: chaliy

crates/bashkit/src/parser/lexer.rs

@@ -326,6 +326,10 @@ impl<'a> Lexer<'a> {
                         self.advance();
                         if let Some(next) = self.peek_char() {
                             match next {
+                                '\n' => {
+                                    // \<newline> is line continuation: discard both
+                                    self.advance();
+                                }
                                 '"' | '\\' | '$' | '`' => {
                                     word.push(next);
                                     self.advance();
@@ -578,7 +582,11 @@ impl<'a> Lexer<'a> {
                     if let Some(next) = self.peek_char() {
                         // Handle escape sequences
                         match next {
-                            '"' | '\\' | '$' | '`' | '\n' => {
+                            '\n' => {
+                                // \<newline> is line continuation: discard both
+                                self.advance();
+                            }
+                            '"' | '\\' | '$' | '`' => {
                                 content.push(next);
                                 self.advance();
                             }

crates/bashkit/tests/spec_cases/bash/variables.test.sh

@@ -408,3 +408,43 @@ x=hello; echo ${x@A}
 ### expect
 x='hello'
 ### end
+
+### var_line_continuation_dquote
+# Backslash-newline inside double quotes is line continuation
+echo "hel\
+lo"
+### expect
+hello
+### end
+
+### var_line_continuation_dquote_var
+# Line continuation in double-quoted variable assignment
+x="abc\
+def"; echo "$x"
+### expect
+abcdef
+### end
+
+### var_line_continuation_dquote_multi
+# Multiple line continuations
+echo "one\
+two\
+three"
+### expect
+onetwothree
+### end
+
+### var_line_continuation_unquoted
+# Backslash-newline in unquoted context
+echo hel\
+lo
+### expect
+hello
+### end
+
+### var_line_continuation_preserves_other_escapes
+# Backslash before non-newline in double quotes is preserved
+echo "hello\tworld"
+### expect
+hello\tworld
+### end

specs/009-implementation-status.md

@@ -103,16 +103,16 @@ Bashkit implements IEEE 1003.1-2024 Shell Command Language. See
 
 ## Spec Test Coverage
 
-**Total spec test cases:** 1194 (1189 pass, 5 skip)
+**Total spec test cases:** 1199 (1194 pass, 5 skip)
 
 | Category | Cases | In CI | Pass | Skip | Notes |
 |----------|-------|-------|------|------|-------|
-| Bash (core) | 833 | Yes | 828 | 5 | `bash_spec_tests` in CI |
+| Bash (core) | 838 | Yes | 833 | 5 | `bash_spec_tests` in CI |
 | AWK | 96 | Yes | 96 | 0 | loops, arrays, -v, ternary, field assign, getline, %.6g |
 | Grep | 76 | Yes | 76 | 0 | -z, -r, -a, -b, -H, -h, -f, -P, --include, --exclude, binary detect |
 | Sed | 75 | Yes | 75 | 0 | hold space, change, regex ranges, -E |
 | JQ | 114 | Yes | 114 | 0 | reduce, walk, regex funcs, --arg/--argjson, combined flags, input/inputs, env |
-| **Total** | **1194** | **Yes** | **1189** | **5** | |
+| **Total** | **1199** | **Yes** | **1194** | **5** | |
 
 ### Bash Spec Tests Breakdown
 
@@ -156,7 +156,7 @@ Bashkit implements IEEE 1003.1-2024 Shell Command Language. See
 | test-operators.test.sh | 17 | file/string tests |
 | time.test.sh | 11 | Wall-clock only (user/sys always 0) |
 | timeout.test.sh | 17 | |
-| variables.test.sh | 58 | includes special vars, prefix env, PIPESTATUS, trap EXIT, `${var@Q}` |
+| variables.test.sh | 63 | includes special vars, prefix env, PIPESTATUS, trap EXIT, `${var@Q}`, `\<newline>` line continuation |
 | wc.test.sh | 35 | word count (5 skipped) |
 | type.test.sh | 15 | `type`, `which`, `hash` builtins |
 | declare.test.sh | 10 | `declare`/`typeset`, `-i`, `-r`, `-x`, `-a`, `-p` |