release: prepare v1.1.0

[Maxerns]

Summary

• test tampered checksums through the helpers used by both installers
• reject malformed, trailing-junk, and duplicate checksum manifest entries
• keep Bash and PowerShell checksum parsing aligned for text and binary manifest lines
• reuse the full action matrix before publishing an exact v1 release
• create the exact release before moving the v1 convenience tag
• prevent concurrent or backwards v1 tag movement
• document exact action releases and the v1 release process

PR #7 delivered the malformed-version and missing-release half of this failure-path work.

Testing

• checked workflows with actionlint
• checked YAML and shell syntax
• tested valid, binary-mode, tampered, malformed, missing-entry, duplicate-entry, and trailing-junk checksum fixtures locally
• forced the macOS shasum fallback (Darwin also provides sha256sum) and exercised malformed checksum fixtures on Linux, macOS, and Windows in GitHub Actions
• reviewed the diff against project standards and the PR 4 spec

[Copilot]

Pull request overview

This PR prepares the v1.1.0 action release by centralizing checksum verification logic (shared by both installers and fixtures), expanding CI coverage for checksum failure paths, and introducing a release workflow that publishes immutable exact tags before updating the moving v1 convenience tag.

Changes:

• Added shared checksum verification helpers for POSIX shell and PowerShell, and updated installers to use them.
• Expanded the test workflow to validate tampered/malformed/duplicate/trailing-junk checksum manifest behavior across OSes.
• Added a release workflow and documentation describing the immutable exact-tag release process and controlled v1 tag movement.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
scripts/verify-checksum.sh	New POSIX checksum helper enforcing single matching manifest entry and digest verification.
scripts/verify-checksum.ps1	New PowerShell checksum helper aligned with the POSIX helper’s parsing/validation intent.
scripts/install.sh	Uses helper script for checksum verification; resolves helper path relative to installer.
scripts/install.ps1	Uses helper script for checksum verification via $PSScriptRoot.
.github/workflows/test.yml	Adds reusable workflow entrypoint and a new tampered/malformed checksum test matrix job.
.github/workflows/release.yml	New release workflow: validates exact tag, reuses test matrix, creates release, then moves v1.
RELEASING.md	Documents the exact-tag release workflow and v1 movement rules/process.
README.md	Documents pinning exact action releases vs using moving v1 and the no-manual-tags policy.
.github/fixtures/checksum/sotto-fixture	Adds checksum test fixture content.
.github/fixtures/checksum/SHA256SUMS	Adds checksum manifest fixture for the checksum helper tests.

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