Add BCR workflow for Bazel Central Registry publishing

.bcr/README.md

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+# Bazel Central Registry (BCR) Publishing
+
+This directory contains configuration files for automatically publishing `fast_float` releases to the [Bazel Central Registry (BCR)](https://github.com/bazelbuild/bazel-central-registry).
+
+## Overview
+
+When a new release is published on GitHub, the workflow defined in `.github/workflows/publish-to-bcr.yml` automatically:
+1. Creates a new entry in the BCR with the release version
+2. Opens a pull request to the [bazel-central-registry](https://github.com/bazelbuild/bazel-central-registry) repository
+3. Allows BCR maintainers to review and merge the update
+
+This makes the new version of `fast_float` available to all Bazel users through the standard module system.
+
+## Files in This Directory
+
+### `metadata.template.json`
+Contains repository metadata including:
+- Project homepage
+- Maintainer information (name, email, GitHub username)
+- Repository references
+
+This information is used to populate the BCR entry's `metadata.json` file.
+
+### `source.template.json`
+Defines how to fetch the source code for each release:
+- URL pattern for release archives
+- Archive strip prefix configuration
+- Integrity hash (auto-generated during publishing)
+
+### `presubmit.yml`
+Specifies the CI tests that BCR will run before accepting a new version:
+- Platforms to test on (Linux, macOS, Windows)
+- Bazel versions to test with
+- Build targets to verify
+
+## How the Publishing Workflow Works
+
+The publishing process is automated through GitHub Actions:
+
+1. **Trigger**: When you create a new release on GitHub (or manually trigger the workflow)
+2. **Build**: The workflow uses [bazel-contrib/publish-to-bcr](https://github.com/bazel-contrib/publish-to-bcr) reusable workflow
+3. **Generate**: Creates BCR entry files from the templates in this directory
+4. **Submit**: Opens a pull request to the Bazel Central Registry
+5. **Review**: BCR maintainers review and merge the PR
+
+## Setting Up the Personal Access Token (PAT)
+
+The publishing workflow requires a GitHub Personal Access Token with appropriate permissions to:
+- Push to a fork of the Bazel Central Registry
+- Open pull requests against the upstream BCR
+
+### Creating a Personal Access Token
+
+1. **Navigate to GitHub Settings**
+   - Go to https://github.com/settings/tokens
+   - Or: Click your profile picture → Settings → Developer settings → Personal access tokens → Tokens (classic)
+
+2. **Generate New Token**
+   - Click "Generate new token (classic)"
+   - Give it a descriptive name, e.g., "BCR Publishing for fast_float"
+   - Set an appropriate expiration (recommended: 1 year with calendar reminder to renew)
+
+3. **Select Required Scopes**
+   - ✅ **`repo`** (Full control of private repositories) - Required for accessing repository details
+   - ✅ **`workflow`** (Update GitHub Action workflows) - Required for the publishing action
+
+4. **Generate and Copy**
+   - Click "Generate token" at the bottom
+   - **Important**: Copy the token immediately - you won't be able to see it again!
+
+5. **Add as Repository Secret**
+   - Go to the fast_float repository settings
+   - Navigate to: Settings → Secrets and variables → Actions
+   - Click "New repository secret"
+   - Name: `BCR_PUBLISH_TOKEN`
+   - Value: Paste the token you copied
+   - Click "Add secret"
+
+### Important Notes
+
+- **Classic PATs vs Fine-grained PATs**: Currently, you must use a "Classic" PAT. Fine-grained PATs don't yet support opening pull requests against public repositories (though GitHub is working on this).
+
+- **Token Security**: Keep the token secure and never commit it to the repository. GitHub Secrets are encrypted and only exposed to authorized workflows.
+
+- **Token Expiration**: Set a calendar reminder before your token expires to generate a new one, otherwise the publishing workflow will fail.
+
+- **Permissions Required**: The token needs both `repo` and `workflow` scopes to function properly.
+
+## Fork Requirements
+
+The publishing workflow requires a fork of the [bazel-central-registry](https://github.com/bazelbuild/bazel-central-registry) repository:
+
+- The fork should be in the repository owner's account or organization
+- The PAT must have write access to this fork
+- The workflow will push changes to the fork and open PRs from there to the upstream BCR
+
+## Manual Triggering
+
+You can manually trigger the publishing workflow:
+
+1. Go to the Actions tab in the GitHub repository
+2. Select "Publish to BCR" workflow
+3. Click "Run workflow"
+4. Enter the tag name (e.g., `v6.1.6`)
+5. Click "Run workflow"
+
+This is useful for:
+- Republishing a release if the automatic workflow failed
+- Publishing an older release that wasn't automatically published
+- Testing the workflow
+
+## Troubleshooting
+
+### Publishing Workflow Fails
+
+1. **Check the workflow logs** in the Actions tab for specific error messages
+2. **Verify the PAT** is still valid and has the correct permissions
+3. **Confirm the fork exists** and the PAT has access to it
+4. **Check the template files** in this directory are valid JSON
+
+### Pull Request Not Opening
+
+1. **Verify PAT permissions** - must include `repo` and `workflow` scopes
+2. **Check fork configuration** - ensure the BCR fork exists and is accessible
+3. **Review workflow inputs** - ensure the tag name matches the actual release tag
+
+### BCR Presubmit Tests Fail
+
+1. **Review presubmit.yml** to ensure test configuration is correct
+2. **Check build targets** are valid in the new version
+3. **Verify MODULE.bazel** and BUILD files are properly configured
+4. The BCR maintainers may provide feedback on the pull request
+
+## Updating Configuration
+
+### To Update Maintainer Information
+
+Edit `metadata.template.json` and update the maintainers array.
+
+### To Change Test Configuration
+
+Edit `presubmit.yml` to modify:
+- Platforms to test on
+- Bazel versions to test with
+- Build targets to verify
+
+### To Modify Source Archive Settings
+
+Edit `source.template.json` if the release archive structure changes.
+
+## Resources
+
+- [Bazel Central Registry](https://github.com/bazelbuild/bazel-central-registry)
+- [publish-to-bcr Documentation](https://github.com/bazel-contrib/publish-to-bcr)
+- [Bzlmod User Guide](https://bazel.build/docs/bzlmod)
+- [GitHub PAT Documentation](https://docs.github.com/en/authentication/keeping-your-account-and-data-secure/managing-your-personal-access-tokens)
+
+## Support
+
+If you encounter issues with BCR publishing:
+1. Check the [publish-to-bcr issues](https://github.com/bazel-contrib/publish-to-bcr/issues)
+2. Review [BCR submission guidelines](https://github.com/bazelbuild/bazel-central-registry/blob/main/docs/README.md)
+3. Open an issue in the fast_float repository for project-specific problems

.bcr/metadata.template.json

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+{
+    "homepage": "https://github.com/fastfloat/fast_float",
+    "maintainers": [
+        {
+            "email": "daniel@lemire.me",
+            "github": "lemire",
+            "name": "Daniel Lemire",
+            "github_user_id": 391987
+        }
+    ],
+    "repository": [
+        "github:fastfloat/fast_float"
+    ],
+    "versions": [],
+    "yanked_versions": {}
+}

.bcr/presubmit.yml

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+matrix:
+  platform:
+  - debian10
+  - ubuntu2004
+  - ubuntu2204
+  - macos
+  - macos_arm64
+  - windows
+  bazel:
+  - 9.*
+  - 8.x
+  - 7.x
+  - 6.x
+tasks:
+  verify_targets:
+    name: Verify build targets
+    platform: ${{ platform }}
+    bazel: ${{ bazel }}
+    build_targets:
+    - '@fast_float'

.bcr/source.template.json

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+{
+    "integrity": "",
+    "strip_prefix": "{REPO}-{VERSION}",
+    "url": "https://github.com/{OWNER}/{REPO}/archive/refs/tags/v{VERSION}.tar.gz"
+}

.github/workflows/publish-to-bcr.yml

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+name: Publish to BCR
+
+on:
+  release:
+    types: [published]
+
+  # Allow manual triggering from GH UI
+  workflow_dispatch:
+    inputs:
+      tag_name:
+        description: 'Tag name to publish (e.g., v1.2.3)'
+        required: true
+        type: string
+
+permissions:
+  id-token: write
+  attestations: write
+  contents: write
+
+jobs:
+  publish:
+    uses: bazel-contrib/publish-to-bcr/.github/workflows/publish.yaml@v1.0.0
+    with:
+      tag_name: ${{ github.event.release.tag_name || inputs.tag_name }}
+    secrets:
+      publish_token: ${{ secrets.BCR_PUBLISH_TOKEN }}