Introduce immutable runtime snapshots and PCAP replay for v1.4.0-beta.1

[masarray]

Engineering intent

Begin the v1.4 runtime architecture with a coherent immutable publication boundary and deterministic offline PCAP replay through the same decoder/analyzer entry point used by live Npcap capture.

Implemented

• immutable selected-stream runtime generations with copied identity, waveform samples, analog/RMS/phasor values, shape evidence, and diagnostics
• atomic snapshot publisher: consumers see either the previous complete generation or the next complete generation
• AnalyzerRuntimeSnapshotSource adapter so live or other IAnalyzerDataSource implementations can migrate to immutable generations without changing capture contracts
• bounded classic PCAP 2.4 reader for Ethernet link type 1
• little-endian and big-endian PCAP support
• microsecond and nanosecond timestamp variants
• replay routed through RawProcessBusAnalyzer.ObserveOwnedFrame, the same entry point used by live Npcap capture
• deterministic coverage for replay timing, immutable generations, three-stream isolation, truncated records, unsupported link type, invalid timestamp fractions, and frame-size boundaries
• dedicated repeated Runtime Architecture workflow with TRX evidence
• candidate-package support for architecture/* branches
• version, release workflow, landing page, changelog, roadmap, packaging guide, tested matrix, release checklist, release notes, and issue form synchronized to 1.4.0-beta.1

Automated validation

• Repository health gate
• Clean restore and Release build
• Full test suite: 54/54 passed
• Runtime Stability workflow
• Runtime Architecture: 3 repeated passes, 14/14 tests per pass
• CodeQL
• NuGet high/critical vulnerability audit
• GitHub dependency delta review
• Portable candidate publish and package verification
• Candidate checksum and manifest

Candidate commit: 1461584b2d90b0bde1c472728b166cadd924d366

Portable candidate SHA256:
a251dbcd2f94c6a4e60d2c207263b188be316c4c6110ffb022916b84843cf35c

Safety boundary

• receive-only behavior remains unchanged
• replay is file input only and never publishes SV/GOOSE traffic or sends control commands
• replay uses the existing decoder instead of introducing a second protocol implementation
• Windows/Npcap and recorded timestamps remain screening evidence unless independently validated

Honest scope limits

• classic PCAP is supported; PCAPNG is not yet claimed
• the immutable boundary and live-source adapter are implemented, but the WPF live workspace has not yet been fully migrated to the new runtime generation model
• the internal analyzer is still partly monolithic; complete SvStreamRuntime, sequence tracker, windowing, phasor, and shape-component extraction continues in later v1.4 builds
• replay does not recreate switch loading, NIC buffering, packet loss absent from the capture, or hardware timestamp behavior

Remaining release gate

• Windows/Npcap regression smoke using the candidate package
• confirm existing live SV/GOOSE/PTP/SCL behavior did not regress
• optional sanitized classic-PCAP replay verification through an engineering harness/API consumer
• final maintainer release decision

Keep this PR in draft until the candidate field regression is complete.