Add optional variables and fix stdout/stderr ordering

[andreasjansson]

Two changes:

1. Optional variable modifier for test patterns

Add support for optional variables that mark output lines which may or may not appear.

Syntax

{{ var: optional string }}       # optional with explicit type
{{ var: optional number }}       # optional number
{{ var: optional json object }}  # optional JSON types
{{ var: optional }}              # optional with duck typing

An optional variable must occupy an entire line by itself in the expected output pattern. When the actual output doesn't contain that line, the test still passes and the variable is not captured. When present, the variable captures normally and can be used in constraints.

Example

===
test with optional progress line
===
./my-command
---
{{ progress: optional string }}
result: {{ value: number }}
---
where
* value > 0

This matches both result: 42 (no progress line) and Processing...\nresult: 42 (with progress line). Multiple consecutive optional lines are also supported.

Implementation

• Parser (cctr-corpus): Extract optional prefix in parse_placeholder, added optional: bool field to VariableDecl
• Matcher: build_regex wraps optional-only lines in (?:...)? groups with correct newline handling for start/middle/end positions
• 10 matcher unit tests, 5 parser unit tests, 18 corpus tests

---

2. Fix stdout/stderr stream ordering

Previously run_command read all of stdout then appended all of stderr:

let combined = format!("{}{}", stdout_str, stderr_str);

This caused stderr to always appear after stdout regardless of when it was actually written. Now uses the same channel-based interleaving approach as run_command_streaming to preserve temporal ordering of output lines from both streams.

6 corpus tests verify ordering with stderr-first, stdout-first, interleaved, and timed delay scenarios.