Reject unsupported output extensions

[eneskemalergin]

Hello there,

I wanted to open this PR because I realized that the output filename can currently conflict with the content Kuva writes. For example, -o plot.pgn, an output path without an extension, and even -o plot.PNG all succeed but contain SVG. The first two can hide a typo, while the last one looks like a PNG but isn't one.

I changed the CLI so .svg, .png, and .pdf are recognized case-insensitively. Other or missing extensions now return an error during argument parsing, before Kuva reads the input or renders anything, and no misleading output file is created.

What I changed:

• added one shared output-format classifier rather than repeating the rule in each plot command;
• used the same classifier for CLI validation and backend selection;
• added regression tests for unknown and missing extensions, validation timing, and mixed-case SVG, PNG, and PDF output;
• updated the related help text, README, CLI documentation, and changelog entry.

I kept the scope limited to CLI file output through -o/--output. SVG on stdout is unchanged, supported lowercase paths behave as before, and PNG/PDF still require their existing features. This does not add a --format flag or change the library backend APIs, terminal output, input handling, or rendering itself.

For testing, I first ran the new behaviour tests against the unchanged dev implementation and confirmed that the invalid paths were accepted and mixed-case PNG/PDF received SVG. On the updated branch, I ran:

• the focused output classifier and CLI tests with both cli and cli,full features;
• cargo ci-fmt;
• cargo ci-clippy;
• cargo ci-test, including the full CLI suite and doctests;
• manual help, failure-side-effect, and --emit-code checks.

All of these passed. I could not run the mdBook build because mdbook is not installed in my local environment, but I reviewed the Markdown changes directly as the source.

I have a few questions where I would especially appreciate your preference:

1. Does strict validation for unknown or missing extensions match the behaviour you want, or would you prefer to keep the implicit SVG fallback?
2. I updated the directly related help, README, CLI documentation, and changelog so they match the new behaviour. If you prefer to keep documentation wording and style under your control, I am completely happy to reduce this PR to the code and tests.

Thank you for taking a look.

Type of change

• New plot type
• New feature / API addition
• Bug fix
• Documentation / assets only
• Refactor / housekeeping

---

Checklist

Library (new plot type)

Not applicable — this does not add or change a plot type or public library API.

Tests

• Added focused unit coverage for supported, mixed-case, missing, unsupported, trailing-dot, and non-Unicode extensions
• Added CLI coverage for failure diagnostics, absence of output side effects, validation before input reading, and SVG/PNG/PDF content
• Focused tests pass with both cli and cli,full
• cargo ci-test — the complete suite passed, including 80 CLI tests and 186 doctests
• cargo ci-fmt
• cargo ci-clippy

CLI (if applicable)

• Shared BaseArgs::output validation and help text updated
• Shared backend selection uses the same format classifier
• tests/cli_basic.rs updated with output and content verification
• docs/src/cli/index.md updated with the new output contract
• man/kuva.1 scope checked — flattened subcommand output options are not present, so regeneration produces no relevant change
• Smoke-test scope checked — no command dispatch or rendering behaviour changed, so no new invocation was added

Documentation

• README.md output guidance updated
• docs/src/cli/index.md output guidance updated
• CLI --help text updated and inspected
• mdBook build — mdbook is not installed in my local environment; changed Markdown was reviewed as source

Visual inspection

Not applicable — this PR does not change plot rendering or layout. Mixed-case PNG and PDF output was validated structurally in the CLI tests.

Housekeeping

• CHANGELOG.md entry added under ## [Unreleased]
• Final diff and failure-side-effect checks reviewed

Enes K.

[Psy-Fer]

I would probably move this under a ### Changed heading rather than fixed, as it causes a break in expectation for someone writing to a output.dat and expecting svg or whatever.

[Psy-Fer]

This is great, and I agree with strict enforcement. Doc changes are good too, just the one thing for the CHANGELOG moving it to a ### Changed subheading over fixed

[Psy-Fer]

Which I fixed doing the merge conflict :)