ci+docs: CI matrix & coverage badges, NuGet release badges

[monsieurleberre]

Mirrors the badge work from daml-codegen-csharp#9, adapted for this single-language repo.

CI workflow (.github/workflows/ci.yaml)

• Add artifact-prefix: csharp to the csharp-ci call so per-run artifacts (coverage value, shard results) are namespaced.
• Add an assemble-badge-data job that tags the csharp-ci matrix-status output with lang: "csharp".
• Add a badges job (on main pushes only, contents: write) calling the shared update-badges.yaml@v2 reusable workflow to publish per-shard CI badges and the coverage badge to the orphan badges branch.

README

• Replace the stale static Release-v0.1.4 badge (and its now-incorrect "repo is private" comment) with dynamic latest stable / latest preview release badges — the repo is public now, so shields.io's github/v/release/... endpoint resolves.
• Add a CI workflow-status badge.
• Add a Platform Support matrix: one badge per OS × arch shard (Ubuntu / Windows / macOS × amd64 / arm64 — all six are supported here, including windows-11-arm).
• Add a Coverage badge.
• Add latest stable / latest preview release badges to the Packages section.

No per-language sub-sections — this repo ships only C#, so the badge groups are flat.

Note

The shields endpoint badges (matrix + coverage) read from the badges orphan branch, which is created/populated on the first main run after this merges. Until then they render as "invalid" — expected.

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C# coverage

Package	Line Rate	Branch Rate	Complexity	Health
Canton.Ledger.Auth	100%	98%	62	✔
Canton.Ledger.Grpc.Client	97%	83%	190	✔
Canton.Ledger.Pqs.Client	84%	88%	54	✔
Daml.Runtime.Grpc	99%	99%	82	✔
Summary	96% (924 / 966)	89% (327 / 366)	388	✔