A repo-wide cleanup pass for professionalism / licensing hygiene:

## Apache 2.0 license header

Prepended the standard Firefly Software Foundation Apache 2.0 header
to every C# source file in src/, samples/, and tests/ — 344 files in
total, all of which previously lacked a header. The text is the same
notice used in the Java sources, normalised to C# `//` line comments:

    // Copyright 2024-2026 Firefly Software Foundation
    //
    // Licensed under the Apache License, Version 2.0 (the "License");
    // you may not use this file except in compliance with the License.
    // You may obtain a copy of the License at
    //
    //     http://www.apache.org/licenses/LICENSE-2.0
    //
    // Unless required by applicable law or agreed to in writing, software
    // distributed under the License is distributed on an "AS IS" BASIS,
    // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
    // implied. See the License for the specific language governing
    // permissions and limitations under the License.

The header sits above the first using directive, separated by a blank
line. Insertion was idempotent (skipped any file already containing
"Firefly Software Foundation"); re-running on top is a no-op. Build:
0 errors / 0 warnings; 346/346 tests still pass.

## Documentation index

Removed `docs/AUDIT.md` (Java↔.NET parity audit) per the user's
direction — the same information now lives implicitly in the README
quickstarts, the per-project READMEs, and the new index pages added
in this PR. All references to AUDIT.md are stripped from README.md.

Added three new top-level README index pages:

* `docs/README.md` — indexes ARCHITECTURE / MODULES / SERVICE-SCAFFOLDING
  / CONFIGURATION / MIGRATION-GUIDE with audience tags and a recommended
  reading order. Replaces the navigation role AUDIT.md served.

* `src/README.md` — lists every src/* project organised by tier
  (Foundational / Platform / Adapters / Starters), with a one-line
  description of each and a link to its own README. Calls out the
  conventions every project follows: Apache header, file-scoped
  namespaces, `<IsPackable>true</IsPackable>` default, options + DI
  extension naming, and the no-silent-stubs rule.

* `tests/README.md` — describes the single-test-project layout, how
  to run filtered subsets, the three test categories (framework
  internals / adapter shape / extras), and the conventions for new
  tests (WireMock for HTTP adapters, NSubstitute for SDK-fronted
  ones, xUnit-native asserts).

## README touch-up

Updated the top-level `README.md` to reflect the new structure:

* "Repository layout" tree shows the three new index READMEs and
  drops the AUDIT.md line.
* "Documentation" section points at `docs/README.md` as the
  recommended starting point and removes the AUDIT.md row.
* Per-project + index READMEs are explicitly cross-linked so a
  reader landing in any of `src/`, `samples/`, `tests/`, or `docs/`
  finds the right next page within one click.

## Verification

* `dotnet build FireflyFramework.sln` — 0 errors / 0 warnings.
* `dotnet test FireflyFramework.sln --no-build` — 346 / 346 passing.
* `find src tests samples -name '*.cs' -exec grep -L 'Firefly Software Foundation' {} \;` — empty (every file has the header).
* `head -2 src/FireflyFramework.Cqrs/Commands/ICommand.cs` shows the
  header sits above the first `using` directive, not inside it.