TD-ADOC-002: Include Options and Attribute Substitution in Paths

## Description
Technical debt from Issue #3 (AsciiDoc Parser): Some include-related features were deferred.

## Remaining Features

### 1. Include Option `leveloffset`
- Syntax: `include::file.adoc[leveloffset=+1]`
- Adjusts heading levels of included file
- Example: `== Chapter` becomes `=== Chapter` with `leveloffset=+1`

### 2. Include Option `lines`
- Syntax: `include::file.adoc[lines=5..10]`
- Includes only specific lines
- Variants: `lines=5..10`, `lines=5;7;9`, `lines=5..-1`

### 3. Attribute Substitution in Include Paths (AC-ADOC-09)
- Syntax: `include::{includedir}/partial.adoc[]`
- Attributes like `{includedir}` should be substituted in paths

## Acceptance Criteria
- [ ] AC-ADOC-09: Attribute substitution in include paths works
- [ ] `leveloffset` option is correctly applied
- [ ] `lines` option filters correctly

## References
- Issue #3: AsciiDoc Parser Core Implementation
- `src/docs/spec/05_asciidoc_parser.adoc`

## Priority
Low - Base parser works without these features

Start with src/docs/spec/05_asciidoc_parser.adoc and the AsciiDoc Parser Core Implementation from Issue #3, then trace the existing include support. Done means attribute substitution in include paths, leveloffset heading adjustment, and all listed lines variants satisfy the acceptance criteria.

Written by the indexing model from the issue text.