Add opt-in build phase progress producer

[Saipriya-1144]

Summary

Adds a minimal, opt-in Oryx build-phase producer for the Kudu Deployment Progress Tracker V1. Oryx remains independent of Kudu persistence, journal, snapshot, and API models: it only emits bounded machine phase signals while the local build script runs.

This is a new implementation from current main. PR #3092 was used only for test and integration lessons; this PR does not modify, retarget, close, or build on it.

Protocol

• Opt-in environment: ORYX_PROGRESS_ENDPOINT=file:<absolute-path> and ORYX_PROGRESS_OPERATION_ID=<opaque-safe-id>.
• Endpoint ownership: the caller must pre-create a regular file outside Oryx source, output, and intermediate cleanup directories. Oryx opens it with FileMode.Open, append-writes, and never creates or deletes it.
• Record format: UTF-8 JSONL with exactly schemaVersion, eventType, operationId, phase, and timestampUtc.
• Event: only phase_started, with schemaVersion: 1.
• Truthful shared phases: pre_build, build.execute, post_build, output.prepare, output.compress, and manifest.write, emitted only when the corresponding existing boundary runs.
• Bounds: 1,024 bytes per record, 32 KiB total file size, and 256 ASCII-safe bytes for the operation ID.
• Fail-open behavior: configuration, serialization, file, locking, or writer-process failures permanently disable reporting for that build process. Writer invocations are silent and capped at two seconds.

Example record:

{"schemaVersion":1,"eventType":"phase_started","operationId":"operation-123","phase":"build.execute","timestampUtc":"2026-10-09T01:02:03.4567000Z"}

Scope and non-scope

The implementation is centralized in the shared base build template plus one hidden internal append command. It does not add capability discovery, build_started, build_completed, outcomes, percentages, deployment IDs, step state, attempts, customer-facing text, commands, paths, logs, exceptions, package/project names, or any Kudu-owned model.

Validation

• dotnet build src\BuildScriptGeneratorCli\BuildScriptGeneratorCli.csproj --configuration Release --no-restore --verbosity minimal — passed; only existing analyzer/obsolete-option warnings.
• dotnet test tests\BuildScriptGenerator.Tests\BuildScriptGenerator.Tests.csproj --configuration Release --no-restore --logger "console;verbosity=minimal" — 599 passed.
• dotnet test tests\BuildScriptGeneratorCli.Tests\BuildScriptGeneratorCli.Tests.csproj --configuration Release --no-restore --logger "console;verbosity=minimal" — 112 passed, 5 existing platform skips.
• Linux WSL self-contained CLI validation — append succeeded with the exact five-field JSONL shape; a missing endpoint was not created. The minimal WSL image lacked ICU, so this validation used DOTNET_SYSTEM_GLOBALIZATION_INVARIANT=1.
• Git Bash integration coverage verifies disabled parity, permanent disable after the first writer failure, unchanged stdout/stderr, xtrace privacy, phase ordering, and generated Bash syntax.

• The purpose of this PR is explained in this message or in an issue. If an issue please include a reference as #<issue_number>.
• Tests are included and/or updated for code changes.
• Proper license headers are included in each file.