bug: backslash line continuation (\\newline) fails in some contexts

[chaliy]

Description

Backslash line continuation (\ at end of line) fails to parse in certain contexts, causing expected 'then' or expected 'fi' errors for valid bash scripts.

Reproduction

# Multi-line if condition (fails)
if [ -z "$A" ] || [ -z "$B" ] || \
   [ -z "$C" ]; then
    echo "missing"
fi
# parse error at line 1, column N: expected 'then'

# Multi-line command with continuation (fails)
az cognitiveservices model list --location "eastus" \
    --query "foo" \
    --output table
# parse error: expected 'fi' (when inside if block)

Source: skills.sh scripts

Found while running real-world skill scripts through bashkit parser:

• microsoft-foundry/generate_deployment_url.sh line 67: multi-line if [ ] || \ condition
• microsoft-foundry/query_capacity.sh lines 26-28: multi-line az command with \ continuation
• microsoft-foundry/discover_and_rank.sh lines 20-23, 26-30: multi-line az rest calls

Cross-ref: specs/015-skills-analysis.md, tests/skills_tests.rs (parse tests pass after removing continuations)

Current behavior

The lexer has line continuation handling at lexer.rs:343, lexer.rs:489, lexer.rs:625 but it doesn't work in all token contexts. Continuation inside if conditions and inside $() command substitution appears broken.

Workaround

Join multi-line commands onto a single line (remove \ + newline). All tests pass after this transformation.