Refactor POS printers architecture

[KickNext]

Summary

• Add a domain-first Dart facade with receipt paper and TSPL media helpers while keeping legacy width-based methods compatible.
• Keep Pigeon as the internal transport layer and add a typed TSPL media host API for physical label geometry.
• Split Android rules into focused domain helpers for status mapping, TSPL layout, UDP config, and bounded TCP discovery.
• Refresh README, doc architecture notes, TSPL guide, example app, changelog, and license.

Verification

• flutter analyze
• flutter test
• ./gradlew :pos_printers:testDebugUnitTest from example/android
• flutter build apk --debug from example
• dart pub publish --dry-run (0 warnings)

Notes

• Flutter/Gradle still prints future built-in Kotlin migration warnings. Full migration is intentionally not included because the local Flutter toolchain is not yet on the required built-in Kotlin migration path.

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 07feeeb18c

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[chatgpt-codex-connector]

Scan local-subnet hosts before enforcing host cap

This loop always starts at the beginning of the CIDR range and stops after maxHosts entries, which means large networks (for example a /16 with local IP 192.168.100.x) only probe 192.168.0.1-192.168.0.254 and never reach the segment where the printer likely resides. Because TcpPrinterDiscovery now defaults to a 254-host cap, TCP discovery can miss reachable printers on valid enterprise/VPN subnets that were previously discoverable when scanning the full range.

Useful? React with 👍 / 👎.