Add optional zero-margin screenshot layout

[thomasrudolfschneider]

Summary

Adds an opt-in zero-margin mode for screenshot grids.

The new flag:

--screenshots_zero_margin

removes outer margins and spacing between screenshot panels. The existing
layout remains unchanged by default.

The Python API exposes the same behavior through
screenshots_zero_margin=False, and createScreenshots() through
ZERO_MARGIN=False.

Backward compatibility

• disabled by default;
• existing screenshot output is unchanged unless explicitly enabled;
• existing public positional parameters retain their indices;
• no new dependency is required.

Validation

• default rendering unchanged;
• zero-margin 12 x 3 screenshot grid visually verified;
• ruff check . passes;
• codespell passes;
• Sphinx documentation build with -W succeeds.

Closes #112

[Copilot]

Copilot review overview

🟡 Changes recommended

Automatic tight layout can override the requested bounds, and the option is not propagated to all screenshot-producing modules.

Review effort: Balanced
Findings: 2

Open (2)

• Disable tight layout when explicit zero-margin bounds are applied · New
• Propagate pipeline setting to fornix, hypothalamus, and hippocampus grids · New

What changed in this PR

Adds opt-in zero-margin rendering for screenshot grids while preserving default layouts.

Changes:

• Adds CLI and Python API configuration.
• Applies explicit zero subplot margins and spacing.
• Documents the new option.

File	Description
README.md	Documents the CLI flag.
fsqc/​fsqcMain.py	Parses and propagates the setting.
fsqc/​createScreenshots.py	Implements zero-margin layout.
doc/​Usage.rst	Updates usage documentation.
DESCRIPTION.md	Adds the option to the command reference.

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