(#155) Cake v6.0.0 catch-up + IEnumerable<string> overloads

[gep13]

Closes #155.
Closes #2.

Scope

Cake 6 catch-up (#155)

• Addin: Cake.Core / Cake.Common 5.0.0 → 6.0.0. TFMs gain net10.0 (now net8.0;net9.0;net10.0).
• Tests: Cake.Testing 5.0.0 → 6.0.0. Tests TFM stays at net8.0 (lowest-of-addin's-range, per the workspace rotation table for Cake 6.x).
• global.json: SDK 9.0.100 → 10.0.100, matching the new highest TFM.
• demo/script/.config/dotnet-tools.json: cake.tool 5.1.0 → 6.1.0 (latest 6.x as of 2026-05-07).
• demo/frosting/build/Build.csproj: Cake.Frosting 5.1.0 → 6.1.0. HintPath stays at net8.0.
• demo/sdk/cake.cs (NEW): Cake.Sdk@6.1.1 file-based program with #:project ../../src/Cake.FileHelpers/Cake.FileHelpers.csproj. Mirrors the alias surface exercised by demo/script and demo/frosting. No Nullable=enable / NoWarn=CS8603 properties needed — Cake.FileHelpers' public API doesn't use nullable annotations, so the source-generator nullability gap (workspace memory feedback_demo_folder_pattern) isn't triggered.
• build.yml: new Run demo/sdk step after Run demo/frosting, using working-directory: ./demo/sdk and dotnet cake.cs.

IEnumerable<string> overloads (#2)

10-year-old request — fold in as a free modernisation since v9.0.0 is a Breaking-change release boundary anyway. Four new public overloads on FileHelperAliases:

• FileWriteLines(this ICakeContext, FilePath, IEnumerable<string>)
• FileWriteLines(this ICakeContext, FilePath, Encoding, IEnumerable<string>)
• FileAppendLines(this ICakeContext, FilePath, IEnumerable<string>)
• FileAppendLines(this ICakeContext, FilePath, Encoding, IEnumerable<string>)

Existing string[] overloads stay (binary-compat for downstream addins linking via PackageReference); overload resolution picks the existing string[] form when a literal new[] { ... } is passed, so no behaviour change at any current call site. The internal WriteLines helper widens to IEnumerable<string> — string[] satisfies that contract, so all existing call sites keep working unchanged.

Tests added covering each new overload via List<string> typed as IEnumerable<string> (guarantees the new overload is dispatched rather than the string[] one).

Out of scope

• No ArgumentNullException.ThrowIfNull modernisation — Cake.FileHelpers' source has no explicit null-checked code paths (the addin defers null handling to the underlying System.IO calls), so there's nothing to modernise.
• Long-stale feature requests Add overloads to allow specify multiple replacements #5 / Add "Remove ReadOnly-Attribute" method #6 / FileFilterExclude / FileFilterInclude #24 — each needs an API-shape decision before implementation. Tracked as a single triage follow-up in the workspace TODO.
• Renovate PRs and the addin-side IDisposableAnalyzers / Microsoft.SourceLink.GitHub analyzer pins — those track .NET / source-link versions, not Cake major, and Renovate drives them.

Local verification

• dotnet cake recipe.cake --target=Package — green (DotNetCore-Test + DotNetCore-Pack both pass).
• dotnet test --filter "FullyQualifiedName~AcceptsIEnumerable" — 4/4 pass for the new overload tests.
• ./demo/script/build.ps1 — all 8 tasks succeeded under cake.tool 6.1.0.
• ./demo/frosting/build.ps1 — all 8 tasks succeeded under Cake.Frosting 6.1.0.
• dotnet cake.cs (in demo/sdk/) — all 8 tasks succeeded under Cake.Sdk 6.1.1 + .NET 10 SDK 10.0.107.

Release context

• Update Cake.FileHelpers add-in to target Cake v6.0.0 #155 (Breaking change) — the Cake.Core 5.0.0 → 6.0.0 bump.
• FileHelpers.xxxLines Method (String, IEnumerable<String>) #2 (Feature) — the IEnumerable<string> overloads.

Both are attached to milestone 9.0.0; this PR closes both via Closes #155 / Closes #2.

[gep13]

LGTM!