Bump version to 0.4.0

[masa10-f]

Context (if applicable):

Prepare the GraphQOMB v0.4.0 release after the features currently documented under Unreleased landed on master.

Description of the change:

• Update the project version from 0.3.1 to 0.4.0.
• Keep the editable GraphQOMB package version in uv.lock aligned with the project metadata.
• Move the accumulated changelog entries into a 0.4.0 section dated 2026-07-19.

Validation:

• uv lock --check
• Package metadata reports 0.4.0
• uv run --extra stim pytest -q (667 passed)
• git diff --check

Related issue:

None.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.27%. Comparing base (3f6ad4e) to head (161bd2e).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #234   +/-   ##
=======================================
  Coverage   86.27%   86.27%           
=======================================
  Files          29       29           
  Lines        4635     4635           
  Branches      799      799           
=======================================
  Hits         3999     3999           
  Misses        450      450           
  Partials      186      186           

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