hcl2tojson --with-meta flag accepted but produces no metadata

[kkozik-amplify]

Description

echo 'x = 1' | hcl2tojson - --with-meta
# → {"x": 1}
# Expected: {"x": 1, "__start_line__": 1, "__end_line__": 1}

The --with-meta flag is accepted without error but the output contains no __start_line__ / __end_line__ keys.

Root Cause

The with_meta option is a stub that was never wired up during the v8 architecture overhaul:

• cli/hcl_to_json.py defines the flag and passes it to SerializationOptions ✓
• hcl2/utils.py defines with_meta: bool = False on the dataclass ✓
• No serialize() method in rules/base.py ever checks options.with_meta ✗
• The metadata (line, end_line, column, end_column) exists on every LarkRule._meta — it's just never written to the output dict
• hcl2/const.py is missing START_LINE_KEY / END_LINE_KEY constants
• Tests only verify the flag is accepted without error, not that output contains metadata

Expected Behavior

When --with-meta is passed, every attribute and block in the output dict should include __start_line__ and __end_line__ keys sourced from LarkRule._meta.

Workaround

Use hq --with-location which correctly provides line/column metadata.

Suggested Fix

Each serialize() method (AttributeRule, BlockRule, etc.) needs to check if options.with_meta: and inject __start_line__ / __end_line__ keys from self._meta. Add corresponding constants to hcl2/const.py.

Related: #130