feat: add ISupportDryRun interface (#259)

[Chris-Wolfgang]

Summary

Implements #259 — adds the ISupportDryRun interface to Wolfgang.Etl.Abstractions.

public interface ISupportDryRun
{
    bool IsDryRun { get; set; }
}

Opt-in marker honoured by side-effect-bearing stages: when IsDryRun is true the
full pipeline runs (enumeration, skip/max counts, progress counters, timer callback,
logging) but the external side effect that mutates a destination/source is skipped.

Design decisions (recorded on #259)

• Stage-agnostic, not loader-only — also covers extractors that mutate their source as they read (queue ack/commit, delete-on-receipt, move processed file). Pure transformers won't implement it.
• No base-class implementation — the side effect lives inside the derived stage's worker method, which the base class cannot see or skip. Implementing on a base would advertise a guarantee it can't keep. Mirrors the existing IncrementCurrentItemCount philosophy.
• Honour-system enforcement lives in a TestKit contract-test base (Add ISupportDryRun contract-test base (verify stages honor IsDryRun) ETL-Test-Kit#195, depends on this shipping).

Changes

• ISupportDryRun.cs with full XML docs
• PublicAPI.Unshipped.txt updated
• CHANGELOG Unreleased / Added entry
• Version 0.14.1 → 0.15.0 (additive public surface = MINOR)

Verification

• dotnet build -c Release — 0 warnings, 0 errors
• dotnet test -c Release -f net8.0 — 250 passed (no behavioural code added; a bare marker interface has no executable lines)

Stack

Base: vNext. First PR of the 0.15.0 cycle.

Closes #259

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