Unifies mapmov and fixes --outlier in robreg

[m-reuter]

• unifies mapmov and maphdr in CLI
• allows --init-lta in robreg and coreg now
• fixes --outlier output bug in robreg

[Copilot]

Pull request overview

This PR standardizes “mapped moving image” outputs across registration CLIs/backends by introducing shared save helpers (resliced + header-only), and it tightens robreg --outliers behavior when IRLS weights are unavailable.

Changes:

• Add shared image export helpers (save_resliced_r2r_image, save_header_mapped_image) and use them across CLIs/backends for --mapmov/--mapmovhdr.
• Fix/guard robreg outlier-map saving and make CLI output conditional on the file actually being written.
• Update tests to validate linear-mode mapping behavior and output dtype, and bump project version to 0.4.0.

Reviewed changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
neuroreg/image/map.py	Adds shared reslice/header-map save helpers and mode inference; updates mode normalization.
neuroreg/image/__init__.py	Re-exports new image helper APIs.
neuroreg/imreg/gd.py	Switches mapped-image export to shared save helper.
neuroreg/imreg/powell.py	Switches mapped-image export to shared save helper.
neuroreg/imreg/reg_model.py	Changes default interpolation mode to "linear" in map_image.
neuroreg/imreg/irls.py	Ensures first-iteration weights/mask are retained even if error becomes NaN.
neuroreg/imreg/robreg.py	Adds robustness checks before saving outlier maps.
neuroreg/cli/robreg.py	Renames mapped output flag to --mapmov, adds --mapmovhdr, and warns if outlier file not written.
neuroreg/cli/coreg.py	Adds --mapmov/--mapmovhdr and forwards mapped_name into backend.
neuroreg/cli/bbreg.py	Adds --mapmov/--mapmovhdr and writes mapped outputs using shared helpers.
neuroreg/cli/segreg.py	Uses shared helpers and dtype-based interpolation inference when mapping segmentations.
tests/test_cli.py	Adds coverage for --mapmov/--mapmovhdr behavior in robreg and coreg CLIs.
tests/test_coreg.py	Verifies mapped-image writes use mode="linear" via monkeypatching.
tests/test_bbreg.py	Adds coverage for bbreg CLI mapped outputs and expected geometry/dtype.
tests/test_segreg.py	Ensures mapped output from uint8 input is written as float32 in linear mode.
tests/test_robreg.py	Tests _save_outlier_map warning/skip behavior when weights are missing.
tests/test_irls.py	Adds regression test ensuring weights persist even when tracked error is NaN.
pyproject.toml	Bumps version to 0.4.0.

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